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Journal of Bacteriology, September 2000, p. 5036-5045, Vol. 182, No. 18
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Characterization of the Operon Encoding the Alternative sigma B Factor from Bacillus anthracis and Its Role in Virulence

Agnès Fouet,* Olivier Namy,dagger and Guillaume Lambert

Toxines et Pathogénie Bactériennes (URA 1858, CNRS), Institut Pasteur, Paris, France

Received 3 April 2000/Accepted 19 June 2000


    ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

The operon encoding the general stress transcription factor sigma B and two proteins of its regulatory network, RsbV and RsbW, was cloned from the gram-positive bacterium Bacillus anthracis by PCR amplification of chromosomal DNA with degenerate primers, by inverse PCR, and by direct cloning. The gene cluster was very similar to the Bacillus subtilis sigB operon both in the primary sequences of the gene products and in the order of its three genes. However, the deduced products of sequences upstream and downstream from this operon showed no similarity to other proteins encoded by the B. subtilis sigB operon. Therefore, the B. anthracis sigB operon contains three genes rather than eight as in B. subtilis. The B. anthracis operon is preceded by a sigma B-like promoter sequence, the expression of which depends on an intact sigma B transcription factor in B. subtilis. It is followed by another open reading frame that is also preceded by a promoter sequence similarly dependent on B. subtilis sigma B. We found that in B. anthracis, both these promoters were induced during the stationary phase and induction required an intact sigB gene. The sigB operon was induced by heat shock. Mutants from which sigB was deleted were constructed in a toxinogenic and a plasmidless strain. These mutants differed from the parental strains in terms of morphology. The toxinogenic sigB mutant strain was also less virulent than the parental strain in the mouse model. B. anthracis sigma B may therefore be a minor virulence factor.


    INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Bacillus anthracis is the etiological agent of anthrax, a mammalian disease, and is usually regarded as the only pathogen belonging to the Bacillus genus (68). We have studied the response of this bacterium to various stresses by isolating its clpB and clpC genes (57). We have shown that both genes are functional and that the expression of clpB is induced in response to heat shock (unpublished observation). In Bacillus subtilis, the clpC operon is transcribed by a sigma B-containing RNA polymerase (41).

In bacteria, the initiation of transcription is dependent on the sigma factor associated with the RNA polymerase core enzyme. Different promoter specificities are associated with alternative sigma factors and result in a change in the pattern of gene expression. In B. subtilis, various environmental stresses induce the synthesis and activation of sigma B (35). sigma B then initiates the transcription of more than 100 stress genes that constitute the sigma B regulon (6, 11, 25, 35, 75). The activation of sigma B itself involves a network of regulatory proteins (Fig. 1). Seven proteins are involved in this process. They are encoded by rsb genes (for "regulator of sigma B") belonging to the eight-gene sigB operon (8, 12, 22, 39, 79, 84). This network of proteins includes kinases and phosphatases, which transmit signals to an anti-anti-sigma and an anti-sigma factor (39, 79, 81, 84, 86). Depending on the kind of stress encountered, the signals are transmitted to the upstream or downstream switch module (Fig. 1B) (1, 39, 79, 81, 84). The last four genes are preceded by a sigma B-dependent promoter; thus, sigma B increases its own transcription as a consequence of its activation, further inducing the entire sigma B regulon (Fig. 1A) (38, 84).


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  		FIG. 1.   Organization of the B. subtilis sigB operon and current model of sigma B regulation. (A) Schematic diagram of the B. subtilis sigB region. PA is the promoter for the eight-gene operon, and PB is an internal, sigma B-dependent promoter. (B) Schematic diagram of the partner-switching modules. For the function of each protein, see main text and references therein. Arrows indicate activation, and T-headed arrows indicate inhibition. P and K stand for phosphatase and kinase, respectively, and U-act stands for RsbU activator.

Despite improvements in our understanding of the mechanism of expression and activation of sigma B, the physiological function of sigma B has remained unclear. The first surprise came when the first sigma B mutants were constructed and found to have no impairment in growth or sporulation (10, 23). Insertional inactivation of the sigma B-dependent ctc gene leads to a sporulation deficiency at high temperature, indicating that genes from the sigma B regulon have physiological functions (71). One possible reason for the lack of effect of deletion of the sigB gene is that many of the genes controlled by sigma B also have sigma B-independent induction pathways (36). There also appears to be some gene redundancy, and so a lack of transcription of a sigma B-dependent gene can be compensated by expression of a sigma B-independent gene encoding a similar function. Indeed, multiple-mutant strains have been constructed and shown to be impaired in resistance to a given stress (24). Culture conditions have also been devised to investigate the physiological functions of sigma B, and the data obtained indicate that the sigma B regulon confers multiple stress resistance on nonsporulating cells (36, 80). However, the advantages conferred by sigma B on B. subtilis in its natural ecosystem cannot easily be assessed.

One way to investigate the physiological importance of the sigma B regulatory network is to test whether the partner-switching mechanism of signal transduction is widespread. This led to studies of the sigB operon of a closely related soil bacterium, Bacillus licheniformis (14). In this organism, the organization of the sigB operon is identical to that in B. subtilis (Fig. 1A). The sequences of the gene products are also extremely similar, with the least highly conserved being RsbX, for which catalytic activity rather than simple protein-protein interaction is required (14, 78). However, this high conservation does not extend to the sigB operons of all gram-positive bacteria.

The sigB operon has been characterized in several bacteria including two gram-positive species, Listeria monocytogenes, a facultative intracellular pathogenic nonsporulating member of the Bacillaceae, and Staphylococcus aureus, an extracellular pathogen. The aim of such studies is mainly to define the stress response that these organisms develop upon entry into the host, where they encounter hostile environments (e.g., acidic and oxidative shocks). The organization of the sigB operon in L. monocytogenes is somewhat similar to that in B. subtilis (7, 83). It contains the last four genes, at least part of rsbU, and the internal sigma B-dependent promoter. As in B. licheniformis, the least highly conserved sequence is that of RsbX. The regulatory network therefore contains the complete downstream module (Fig. 1B). It lacks part of the upstream module but contains at least the first protein of the cascade, RsbX (Fig. 1B). However, the sigma B mutant is impaired in acid stress resistance and in its response to signals such as high osmotic strength (7, 83). The genes encoding the other regulatory proteins may be located at other chromosomal loci. However, sigma B does not appear to be essential for the spread of L. monocytogenes in an animal model (83).

In S. aureus, only the partners of the downstream module are present, because the sigB operon contains four genes and lacks that for RsbX. There is a sigma B-dependent promoter between the first (rsbU) and second (rsbV) genes (42, 52, 85). The rsbU gene, the first of the operon, differs according to the strain analyzed, and an 11-bp deletion has been detected in a collection strain (43). Depending on the stress imposed, the various strains have similar or different responses (15, 30, 43). sigma B is a major regulator of the stress response and is involved in the same regulatory network as Sar (15, 16, 20). Sar is one of two global regulatory elements that control the synthesis of the extracellular and cell surface proteins involved in S. aureus pathogenesis. However, sigma B mutants do not seem to be less virulent than the wild type (58).

B. anthracis is a sporulating pathogen closely related to B. subtilis and B. licheniformis. We therefore decided to study its sigB operon. Anthrax infection begins after inoculation, ingestion, or inhalation of spores, preventing exposure of the bacteria to stressful conditions immediately after entry into the host (29, 45). Germination is required for establishment of the disease. In the murine inhalation infection model, spores germinate in the alveolar macrophages (33). Fully virulent B. anthracis bacilli are toxinogenic and encapsulated. The toxins and probably also the capsule are synthesized in the macrophage (27, 33). The three toxin genes are located on pXO1, and the proteins responsible for capsule synthesis are encoded by genes carried on pXO2 (31, 50). The regulation of expression of these genes has been thoroughly studied (18, 28, 34, 40, 49, 65, 68, 72, 73). They are expressed during the exponential growth phase in response to signals in the host environment (bicarbonate and temperature). Septicemia occurs later in the development of the disease, when the bacilli are under conditions of nutrient limitation.

In this paper, we report the characterization of the B. anthracis sigB operon and analysis of the regulation of its expression. Deletion mutants were constructed, and the toxinogenic derivative was found to be less virulent than its parental strain.


    MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Bacterial strains, vectors, and culture media. Escherichia coli TG1 (46) was used as a host for the cloning experiments. E. coli HB101 harboring pRK24 (69) was used for mating experiments. The B. subtilis and B. anthracis strains used and constructed in this work are listed in Table 1.

                              
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  		TABLE 1.   Bacillus strains and plasmids used and constructed in this study

For cloning experiments, pUC19 was routinely used (87). For mating experiments, DNA fragments were subcloned from pUC19, in which the initial constructions were made, or were directly cloned and inserted into pATDelta S28 (57) or pAT113 (70). pDL was used for beta -galactosidase assays in B. subtilis or as a source of the bgaB gene (88). More specific plasmids used or constructed in this work are listed in Table 1.

E. coli were cultured in Luria (L) broth or on L agar plates (51). B. subtilis cells were grown in L broth, on L agar plates, or in 121J medium with or without added glucose (55). B. anthracis cells were grown in brain heart infusion (BHI) broth (Difco) or on BHI agar plates, in L broth, or on NBY agar (31). Antibiotics were used at the following concentrations: 100 µg of ampicillin ml-1 and 40 µg of kanamycin ml-1 for E. coli, 100 µg of spectinomycin ml-1 for both E. coli and B. anthracis, 5 µg of erythromycin ml-1 for B. anthracis, and 5 µg of chloramphenicol ml-1 for B. subtilis.

DNA manipulation and sequencing. Methods for plasmid extraction, endonuclease digestion, ligation, and agarose gel electrophoresis were as described by Maniatis et al. (46). PCR amplification and the filling in of the ends of DNA molecules, using Vent DNA polymerase, were performed as indicated by the manufacturer (New England Biolabs). If bacterial colonies were used instead of DNA, the polymerase was added after an initial incubation for 5 min at 100°C. Chromosomal DNA was extracted as described by Delecluse et al. (19). Sequences were determined either from PCR products or from double-stranded DNA by the dideoxy chain termination procedure (62) using Sequenase kits (Amersham/USB) or the PRISM AmpliTaq dye primer sequencing kit (Applied Biosystems) with an Applied Biosystems PRISM 373A sequencer. Nucleotide and deduced amino acid sequences were analyzed using the Wisconsin package (Genetics Computer Group Inc.).

General methods. E. coli cells were made competent as described by Chung and Miller (17). B. subtilis strains were transformed using the method of Kunst and Rapoport (44). Recombinant plasmids were transferred from E. coli to B. anthracis by a heterogramic conjugation procedure (69). Allelic exchange was carried out as described previously (60). Transduction experiments with bacteriophage CP51 were performed as described by Green et al. (31).

Cloning of the sigB locus and disruption of the sigB gene. The initial DNA fragment (about 750 bp) was amplified by PCR using the degenerate oligonucleotides rsbW52 and sigB147 and inserted into pUC19 (Table 2; Fig. 2) (pSB; see also Results). A fragment comprising the insert in pSB was cloned by inverse PCR. Chromosomal DNA was digested with EcoRI, for which there are no known sites in the target sequence, ligated, and used as a template for amplification with the divergent primers rsbW82 and rsbW135 (Table 2; Fig. 2A). The amplicon, a 2.05-kb fragment, was digested with ClaI, immediately 5' to rsbW82, and inserted into pUC19, giving rise to pSigB2 (Fig. 2B). The sequence analysis indicated that the sigB operon was not complete. We decided to clone the genes preceding rsbW by using a direct cloning and selection procedure. Since the SSB10 (Delta sigB) strain had been constructed with an erythromycin resistance cassette inserted into sigB, an erythromycin-resistant clone could be selected after digesting SSB10 chromosomal DNA with an enzyme for which there was a site either within orf4 or 3' to it and no site in either the resistance cassette or the rest of the known sequence. Various restriction enzymes were used alone or in combination (EcoRV, AlwNI, and HpaI). EcoRV digestion gave rise to the 4.9-kb DNA fragment of pOSB10 (Fig. 2B; Table 1).

                              
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  		TABLE 2.   Primers used in the construction of plasmids



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  		FIG. 2.   Schematic diagram of the B. anthracis sigB region. (A) The sigB operon and the following ORF are represented by long arrows indicating the size and direction of transcription of the genes identified from sequence data. The arrowheads at the ends of the dashed lines indicate the position and orientation of binding of the oligonucleotides used for the cloning experiments described in this work. For the sake of clarity, they have been aligned and sometimes duplicated to indicate the fragments obtained using the various pairs. The 1.8 kb 5' and 0.3 kb 3' to the four indicated genes, which were cloned and sequenced from pOSB10, are not represented. (B) Schematic representation of the B. anthracis chromosomal fragments cloned in different vectors during this work. The bgaB and erm cassettes are also represented. The only restriction sites indicated are those used for chromosome walking by inverse PCR: E, EcoRI; C, ClaI.

Delta sigB strains were constructed as follows. A fragment overlapping the 3' end of rsbW and the 5' end of sigB was amplified using rsbW135 and sigB662 as primers (Table 2; Fig. 2A). The fragment was digested with SmaI and inserted into pATDelta S28, giving rise to pRswA4 (Fig. 2B). A DNA fragment overlapping orf4 was amplified with sigB1280 and sigB1953 as primers (Table 2; Fig. 2A), digested with SmaI and BamHI and inserted into pRswA4. Plasmid pON3.12 (Fig. 2B) was digested with SmaI, and an erythromycin cassette was inserted into it, giving rise to pON12 (Fig. 2B; Table 1). The cassette therefore replaces the DNA fragment between oligonucleotides sigB662 and sigB1280. HB101(pRK24) was transformed with pON12, and the transformant was used in mating experiments with B. anthracis 7702 (pXO1+) to produce SSB10, the Sterne Delta sigB derivative. To obtain the plasmidless Delta sigB strain, GSB10, a phage transduction experiment using CP51 was carried out with SSB10 as the donor and 9131 as the recipient (Table 1).

Construction of pgsiB-bgaB, prsbV-bgaB, porf4-bgaB, and sigB-bgaB transcriptional fusions. The gsiB promoter was obtained by digesting pJPM70 with EcoRI and HindIII (55). The 370-bp fragment was blunted and inserted into pDL that had previously been cut with SnaBI, giving rise to pGL100 (Table 1).

pOSB17, harboring the rsbV-bgaB fusion, was constructed by amplifying the rsbV promoter region with primers rsbV-80 and rsbV+20 (Table 2; Fig. 2) and inserting this fragment into pDL digested with SnaBI. The 2.25-kb fragment containing the fusion was purified after digesting pOSB17 with EcoRI and Ecl 136II. This fragment was blunted and inserted into pSAL322 (48), which had been digested with BamHI and treated with Vent DNA polymerase. The resulting plasmid was pOSB27 (Fig. 2B; Table 1).

The orf4-bgaB fusion was constructed by inserting the amplified orf1030-orf1361 fragment (Table 2; Fig. 2A) into pDL digested with SnaBI, giving rise to pGL300 (Fig. 2B). pON30 was obtained by inserting the DNA fragment used to construct pGL300 into pB5 digested with SnaBI (Fig. 2B) (57). pON300 was constructed by inserting the pON30 2.85-kb PvuII fragment into pSAL322 (Fig. 2B). The vector was digested with BamHI, and all the fragments were treated with Vent DNA polymerase before ligation (Table 1).

The pDL derivatives were used to transform B. subtilis SMY and QB4919 (Table 1). The corresponding inserts were integrated into the chromosome within the alpha -amylase gene by double crossover. The inactivation of the alpha -amylase gene was demonstrated by the absence of a halo of starch hydrolysis on TBAB (Difco)-starch plates stained with 1% iodine.

The pSAL322 derivatives were transferred by mating into B. anthracis strains (Table 1). The corresponding inserts were integrated into the chromosome within the eag gene by double crossover. Integration was demonstrated by the loss of the erythromycin resistance provided by the vector and was checked by appropriate PCR amplifications.

The nondisruptive sigB-bgaB transcriptional fusion, inserted into the sigB locus, was constructed as follows. A DNA fragment was amplified using sig266 and sig1238 as primers (Table 2; Fig. 2A), digested with EcoRI and KpnI, and ligated into pATDelta S28, giving rise to pSBG2 (Fig. 2B). The bgaB gene was extracted from pDL by KpnI-Ecl136II double digestion and inserted into pSBG2 digested with KpnI and SmaI. The resulting plasmid carrying the fusion was called pSBG4 (Fig. 2B). To construct pSBG10, the erm gene and orf4 were simultaneously amplified using sig266 and sig1953 as primers and pON12 as template (Fig. 2; Table 1). The amplicon was inserted into pSBG4 that had been digested with BamHI and blunted. The orientation of the insert (sigB-bgaB-erm-orf4) was checked by PCR. pSBG10 was then transferred into B. anthracis 9131, and correct insertion by double crossover into sigB and orf4 was checked (GSB1) (Table 1).

Enzyme assay. beta -Galactosidase activity was determined as described by Dingman et al. (21), except that the assay temperature used was 55°C instead of 37°C. The protein concentration was determined using the bicinchoninic acid protein assay reagent (Pierce). The curves show results from a typical experiment; each experiment was carried out at least three times.

Infection of mice. Pathogen-free 6-week-old female Swiss mice were supplied by IFFA-CREDO. Groups of 10 mice were subcutaneously injected with different spore doses (104 to 108) of strain 7702 or SSB10, and mortality was monitored as described previously (32).

Nucleotide sequence accession number. The sequence in this paper has been deposited under accession number AJ272497.


    RESULTS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

Cloning of the sigB locus from B. anthracis. We first looked for well-conserved amino acid sequences in RsbW and sigma B from B. subtilis (10, 23) and S. aureus (85). Using published Bestfit comparisons, we identified residues 52 to 61 for RsbW and 147 to 156 for sigma B (S. aureus numbering) and used these sequences to design degenerate oligonucleotides (rsbW52 and sigB147 [Table 2]) (85). If the two sequences were not identical for a particular residue, we used the residue from the B. subtilis sequence because B. anthracis is phylogenetically closer to this organism.

We used these primers to amplify and clone an initial DNA fragment, giving rise to pSB (see Materials and Methods). Sequence analysis indicated that the correct fragment had been isolated. The closest matches for the two incomplete open reading frame (ORF) products were with B. subtilis RsbW and sigma B, respectively. Inverse PCR was successfully carried out with oligonucleotides rsbW82 and rsbw135 to expand the isolated region (pSigB2; see Materials and Methods). pSigB2 starts 30 bp 5' to rsbW and ends 280 bp 3' to orf4 (Fig. 2B). We were unable to clone the 5' sequence of the sigma B operon using this approach. An erythromycin resistance cassette was therefore introduced into the sigB sequence, replacing the DNA fragment located between oligonucleotides sig662 and sig1280 (Fig. 2A, pON12; Fig. 2B, SSB10 [see Materials and Methods]). Using a restriction enzyme that did not cut the known sequence, we cloned a fragment covering the entire region (Fig. 2B, pOSB10 [see Materials and Methods]). pOSB10 contains the four genes shown in Fig. 2 and also approximately 1.8 kb 5' to rsbV and 280 bp 3' to orf4.

Sequence analysis for the B. anthracis sigB locus. The sequence of pOSB10 was determined and analyzed (Fig. 2). Since the completion of this part of the work, The Institute for Genomic Research (TIGR) has begun sequencing the B. anthracis genome. We regularly compared our sequence with their data and found that the contigs identified by the BLASTN search are 100% identical to the sequence overlapping the four genes presented. The DNA sequence of the initial fragment harbored three ORFs that could be organized into an operon. A BLASTP (version 2.0.10) search was carried out with each translation product (5). The first, 112 amino acids long, hereafter referred to as RsbV, was most similar to the S. aureus and B. subtilis RsbV factors (E values, 4 × 1019 and 2 × 1018, respectively). Similarly, the predicted product of the second ORF, a 161-residue polypeptide, was most similar to B. subtilis and B. licheniformis RsbW factors (7 × 1047 and 3 × 1046), and that of the third ORF, a deduced 257-amino-acid protein, was most similar to L. monocytogenes and B. subtilis sigma B (2 × 1074 and 6 × 1073). ORF2 and ORF3 were therefore called rsbW and sigB. As expected, from the high level of similarity between the sequences of the proteins encoded by B. subtilis sigB and sigF, the three deduced amino acid sequences showed various levels of similarity to the products of the sigF operon (E values, 3 × 1012, 2 × 107, and 7 × 1033 for SpoIIAA, SpoIIAB, and sigma F, respectively; 23 to 33% identity and 51 to 60% similarity). The rsbV ORF is preceded by a consensus B. subtilis sigma B recognition sequence (GTTTAA 13 bp GGGTAa) (35, 67).

No putative ORF was found immediately 5' to rsbV. In fact, there are multiple translation stop codons in all frames covering the 600 bp preceding rsbV. Furthermore, the ORF downstream from sigB showed no similarity to B. subtilis rsbX. Thus, unlike the B. subtilis sigB operon, which contains eight genes with an upstream sigma A consensus sequence and an internal sigma B-dependent promoter, the sigB operon of B. anthracis has only three genes, with a single putative promoter (84). The absence of rsbX, whose product acts early in the sigma B regulatory cascade, has already been reported for S. aureus (9, 11, 42, 85). However, in S. aureus, rsbV is preceded by rsbU. A three-gene operon is also encountered in the sporulation factor sigma F-encoding operon of B. subtilis (spoIIA). However, sequence comparisons suggested that the B. anthracis operon studied does not encode sigma F. The second-best matches identified by a BLASTN (2.0a19MP-Wash-U) search with the incomplete TIGR sequence were translated and used to screen SubtiList (4, 53). The second-best match identified for RsbV was SpoIIAA, suggesting that the sigF operon also exists in B. anthracis but is not the operon studied here. No second sigB-like operon was identified, and biological data confirmed that the locus studied was not the sigF operon (see below).

It has been suggested that, for physiological reasons, additional regulators may be encoded elsewhere on the chromosome of S. aureus (14). We therefore searched for equivalents of the rsb genes in the B. anthracis sequence available on the TIGR site, as well as for other homologs as a control (66). We found sequences with high scores for similarity to B. subtilis SpoIIAA, SpoIIAB, and SpoIIE but found no sequences similar to RsbR or RsbS. Thus, the closest match, as expected in the absence of a true homolog, was with SpoIIAA. We also found no sequences similar to RsbT or RsbU; the closest match was, as expected, the end of SpoIIE. We also found no sequence similar to RsbX. Recently, another positive regulator of B. subtilis sigma B, RsbP, has been characterized (74). A BLAST search of the TIGR sequence with this PP2C phosphatase sequence suggested the existence of a homolog in B. anthracis. The sequence identified showed 44% identity and 76% similarity over the 100 central residues (residues 156 to 240) (E value, 4 × 1017). This rather low score may be due to the small size of the contig pulled out (542 nucleotides). A 428-amino-acid homolog of Obg was also identified (E value, 7.4 × 10187). Obg is an essential GTP-binding protein, which is required for the stress activation of B. subtilis sigma B but does not belong to the sigB operon (64).

The product of the ORF just downstream from sigB (designated orf4) is approximately 30% identical and 50% similar (depending on the bacterial origin of the protein [E values, 3 × 106 to 0.001]) to various bacterioferritin proteins. A chromosome-encoded iron capture system has been found in B. anthracis (T. M. Koehler, R. Pasha, and R. P. Williams, Abstr. 92nd Gen. Meet. Am. Soc. Microbiol. 1992, abstr. B-125, 1992). The orf4 gene product is also 27% identical and 40% similar to a nutrient starvation-induced DNA-binding protein (encoded by the dpsA gene) from Synechococcus strain PCC7949 and its homolog from Synechococcus strain PCC6301 (E values, 0.002 and 7 × 104, respectively). This ORF is preceded by a sequence similar to the B. subtilis sigma B consensus recognition sequence (GTTTAA 13bp GGGTAc) (35, 67). The synthesis of the protein encoded by this ORF may therefore be responsive to stress conditions, making it a candidate for membership of the putative B. anthracis sigma B regulon.

The clear difference in sigma B operon organization between B. anthracis and other Bacillus species led us to investigate whether the organization of this operon was unique to this pathogen. We used Southern blotting to analyze the chromosome region harboring sigB in various bacteria from the Bacillus cereus group closely related to B. anthracis, namely, Bacillus thuringiensis (III-BL, III-BS, and subsp. konkukian 97-27) and Bacillus cereus (II4, T6/9778, S8553, and PC1) (37, 59, 61). In all strains tested, including B. anthracis 9131, the same DNA fragment of 5 kb hybridized with the sigB and orf4 probes, obtained by PCR amplification with sig266 plus sig1238 and with sig1280 plus sig1953, respectively. This suggested that there is no other sigB operon in B. anthracis and that a similar chromosomal organization is shared by other closely related organisms. To unambiguously test the absence of rsbX immediately 3' to sigB in these bacteria from the B. cereus group, PCR amplification was carried out on these chromosomal DNAs with convergent oligonucleotides, one internal to sigB and the other one internal to orf4 (orf1030 and orf1361) (Fig. 2A). The same, approximately 300-bp, DNA fragment was obtained in all cases (data not shown). There is therefore no space for rsbX immediately 3' to sigB. All these members of the B. cereus group therefore seem to lack rsbX and probably have a sigB operon similar to that of B. anthracis.

Characterization of a sigB deletion mutant of B. anthracis. The sequence data showed the sigB operon to be the most similar to the operon studied, but the genetic organization of the operon was more like that of the operon encoding sigma F, a sporulation transcription factor. To discriminate between these two possibilities, we constructed mutants in which sigB was deleted and assayed the sporulation efficiency of these mutants. In liquid BHI medium and on NBY agar, the mutants sporulated over the same period as and with similar efficiency to the parental strains. Thus, this operon does not encode a transcription factor that is necessary for sporulation, as sigma F is in B. subtilis.

The sigB deletion mutant and the parental strain differed in morphology. The mutant produced smaller colonies on BHI agar plates, flocculated during growth in liquid medium, and was more difficult to harvest by centrifugation, building up as cotton-like rather than sand-like pellets. These phenotypic differences became clearer with advancing cultures. Optical microscopy showed that the mutant was present as longer filaments than the parental strain. The observed phenotype was very similar to that observed for the S. aureus Delta sigB strain, except for the obvious differences due to one bacterium being a bacillus (long filaments) and the other being a coccus (aggregates) (43). The observed morphological modifications indicated that this gene is usually expressed.

B. subtilis sigma B-dependent expression of two putative B. anthracis promoters. We studied the sigma B dependence of the sigB promoter-like sequences by monitoring their transcriptional response to various environmental conditions in the bacterium in which sigma B was first described (B. subtilis). We made three different constructs. The first, a positive control, contained the B. subtilis gsiB promoter fused to bgaB, which encodes a thermostable beta -galactosidase (55, 56). gsiB responds to multiple stimuli in a sigma B-dependent manner and is one of two genes well characterized as being solely under the control of sigma B (2, 47). In the other two constructs, bgaB was preceded by one of the two B. anthracis sigma B promoter-like sequences, that upstream from rsbV or that upstream from orf4. These constructs, pGL100, pOSB17, and pGL300 (see Materials and Methods) (Table 1), respectively, were integrated into the chromosome of wild-type B. subtilis and of a sigB deletion mutant (GL100, GL200, and GL300, and GLQ100, GLQ200, and GLQ300, respectively [Table 1]). The effect of glucose depletion was then analyzed (Fig. 3). As expected, gsiB was expressed at low levels during exponential growth in medium containing excess glucose and was induced rapidly in response to glucose limitation in the wild-type background (Fig. 3A). No induction was observed if the same experiment was carried out in the Delta sigB background (Fig. 3A). Similar results were obtained with the strains harboring the promoters preceding rsbV and orf4 (Fig. 3B and C, respectively). This indicates that the sequences are efficiently recognized by B. subtilis RNA polymerase and that, like the gsiB promoter, they are dependent on B. subtilis sigma B for their transcription.


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  		FIG. 3.   Expression of beta -galactosidase from pgsiB-bgaB (A), prsbV-bgaB (B), and pORF4-bgaB (C) fusions in parental (squares) and Delta sigB (triangles) B. subtilis strains. Samples were assayed at the times indicated for growth (continuous lines) and for beta -galactosidase activity (dotted lines). The bacteria were cultured in 121J medium (open symbols) and 121J medium from which glucose was depleted at the time indicated by the arrows (solid symbols). OD600, optical density at 600 nm.

Expression of the B. anthracis sigB operon. The morphological changes induced by the deletion of sigB suggested that this gene is normally transcribed in B. anthracis. To confirm this and to study the regulation of expression of the B. anthracis sigB operon, the bgaB gene was inserted between the translational stop codon of sigB and the beginning of orf4 (Fig. 2B, strain GSB1 [see Materials and Methods]). We monitored the transcriptional response of the sigB-bgaB fusion during growth by assessing beta -galactosidase activity (Fig. 4). beta -Galactosidase specific activity increased during the stationary phase, starting shortly after T0 (end of exponential phase). However, this increase in activity was low and persisted throughout the stationary phase (Fig. 4). The highest values reached were consistently those for overnight cultures, with values of 6.5 ± 1 units. To assess the response to stress of this sigma  factor, we subjected the culture to heat shock (Fig. 4). The beta -galactosidase specific activity rose immediately. The transcription of this operon is therefore stress inducible.


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  		FIG. 4.   Expression of beta -galactosidase from a sigB-bgaB fusion in B. anthracis GSB1 in the stationary phase and in response to heat shock. Samples were assayed at the times indicated for growth (continuous lines) and beta -galactosidase activity (dotted lines). Bacteria were cultured in L broth at 37°C (open symbols) or subjected to heat shock (arrow) and then cultured further at 44°C (solid symbols). OD600, optical density at 600 nm.

To determine whether the sigma B consensus sequence upstream from rsbV, the probable promoter of the three-gene operon containing sigB, and the promoter preceding orf4 were indeed B. anthracis sigma B dependent, we constructed two plasmids homologous to those used to assay the rsbV and orf4 promoters in B. subtilis (pOSB27 and pON300 [Fig. 2B; Table 1] [see Materials and Methods]). They were inserted into the B. anthracis chromosome, in the independent eag locus, in the parental strains (9131 and 7702) and sigB-deleted derivatives (GSB10 and SSB10) (forming GSB2, SSB2, SSB3, GSB12, SSB12, and SSB13, respectively [Table 1]). Figure 5 shows the results obtained with SSB3 and SSB13, the strains harboring the orf4 promoter-bgaB transcriptional fusion. The beta -galactosidase specific activity rose during growth, as in GSB1 (Fig. 4), in the parental background, SSB3, but not in the Delta sigB mutant, SSB13 (Fig. 5). This indicates that the orf4 promoter is B. anthracis sigma B dependent. The rsbV promoter was also found to be B. anthracis sigma B dependent from a comparison of the beta -galactosidase specific activity values obtained for late-stationary-phase and overnight cultures in parental (GSB2 and SSB2) and Delta sigB (GSB12 and SSB12) backgrounds (1 and 0.15 U, respectively).


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  		FIG. 5.   Expression of beta -galactosidase from a porf4-bgaB fusion in parental (squares) and Delta sigB (triangles) B. anthracis strains SSB3 and SSB13, respectively, during growth. Samples were assayed at the times indicated for growth (continuous lines) and beta -galactosidase activity (dotted lines). Bacteria were cultured in L broth. OD600, optical density at 600 nm.

In vivo role of the B. anthracis sigma B factor. We injected groups of 10 mice with different doses (104 to 108) of spores of the 7702 strain or its Delta sigB derivative, SSB10. Repeatedly, the number of deaths with given doses of the Delta sigB strain were similar to those obtained with the 1-log-unit lower doses of the parental strain, suggesting a 1-log-unit difference in the 50% lethal dose (LD50). Consequently, for a given dose, the number of deaths was smaller with the Delta sigB strain than with the parental strain. Because there is a certain variability, the determination of a precise LD50 for the Delta sigB strain has been hampered. We have therefore chosen to represent, as an example, the cumulative mortality with a dose equivalent to 1 LD50 for the parental strain (105 spores) for both strains (Fig. 6). Thus, the Delta sigB strain was less virulent than the parental strain. To rule out an effect on toxin syntheses, the in vitro production of protective antigen, i.e., the binding domain common to both toxins, was assayed. It was found to be identical in the mutant and parental strains (data not shown). This is consistent with previous results showing that the three toxin genes are transcribed during the exponential phase of growth, i.e., before the synthesis of sigma B in the absence of stress (65). In addition, no sigma B consensus recognition sequence has been identified upstream from the promoters of the toxin genes (13, 18, 26, 40, 82).


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  		FIG. 6.   Virulence of B. anthracis SSB10 (triangles) and 7702 (squares) strains. Swiss mice were inoculated subcutaneously with 105 spores per mouse (groups of 10 mice). Mortality was recorded daily and plotted as the cumulative number of deaths.


    DISCUSSION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References

In this study, we identified the operon encoding sigma B in B. anthracis. The genetic organization of the sigma B operon is identical in B. subtilis and B. licheniformis and differs from the organization of those in L. monocytogenes and S. aureus, which also differ from one another. In L. monocytogenes, the first four genes are thought to be present because part of the fourth gene (rsbU) has been shown to precede the fifth and because, most importantly, the last, rsbX, whose product belongs to the upstream module, is also present (Fig. 1) (7, 85). rsbX is absent from the S. aureus sigB operon, which contains four genes (42, 83). Since B. anthracis belongs to the genus Bacillus, we thought that its sigB operon would probably be identical to that of the other two Bacillus species studied. In fact, its organization, with three genes, rsbV, rsbW, and sigB, that seem to be conserved in strains from the B. cereus group, is closer to that of the B. subtilis sigF operon than to that of any sigB operon. However, this is not the B. anthracis sigF operon. Our data therefore suggest that neither phylogeny nor physiological similarity (the capacity to sporulate under given growth-limiting conditions) imposes conservation of the genetic organization of the operon encoding the general stress sigma  factor.

We assessed the expression of the studied sigma  factor operon in B. anthracis. To that end, we constructed a sigB deletion mutant. This mutant differed morphologically from the parental strain but sporulated normally. We further analyzed whether this operon encoded a stress response transcription factor by studying the regulation of its expression after imposing stresses on strains containing appropriate transcriptional fusions. Stationary-phase and heat shock inductions of the operon were observed. The integration of fusions between the rsbV or orf4 promoter and a reporter gene, into an independent locus, indicated that the stationary phase-induced initiation of transcription at these promoters was effectively dependent on the B. anthracis sigma  factor, hereafter called sigma B.

The sigma B-dependent, stationary-phase-induced expression of orf4 is of interest. Our data and analysis of the sequences in the vicinity and upstream from the promoter-like sequence of orf4 strongly suggested that this gene was solely under the control of sigma B. In B. subtilis, in which the sigma B regulon has been thoroughly studied, only two genes, gsiB and csbC, have been shown to be good candidates for strict dependence (2, 47). The gsiB gene was isolated because it is induced by glucose starvation. Its product, GsiB, seems to be involved in protection against osmotic stress, and CbsC belongs to a family of proteins containing symporters that transport sugars from the environment (2, 36, 56). The rationale for studying csbC was that elucidation of the regulation and function of strictly sigma B-dependent genes would provide clues to the role of the B. subtilis sigma B regulon (2). Similarly, the function of the orf4 gene product needs to be defined. Weak similarities were found between the sequence of this protein and those of bacterioferritins and nutrient starvation-induced DNA-binding proteins. If the product of orf4 were shown to have the same function as either of these types of protein, this would increase our understanding of anthrax physiopathology.

We found that B. anthracis and B. subtilis sigma B operons do not respond to the same stresses. Glucose starvation could not be achieved because no minimal media from which glucose could be depleted are available for this organism. Stationary phase was induced by addition of azide, but, in contrast to what is described in B. subtilis, this did not induce the expression of the B. anthracis sigma B operon. During noninduced stationary phase, this operon was transcribed later than that of B. subtilis. The sigma B operon of B. subtilis is transcribed from T0 and reaches a steady state around T1 (38). Transcription of the B. anthracis sigma B operon begins, albeit slowly, at the same time point but is still increasing at T5. A similar situation has been described for the S. aureus sigma B operon (42). Analysis of the expression of the B. subtilis sigB operon under various growth conditions, including slow growth, and using various mutants indicated that neither RsbX nor RsbU is required for the energy stress response (3, 63, 76, 77, 80, 81). The sigB induction pattern observed in B. anthracis resembles that described in an rsbU mutant suppressor strain derived from a B. subtilis RsbX- strain (66). In B. subtilis, stationary-phase induction seems to involve a specific RsbV-P phosphatase, RsbP, with sigma B being activated when RsbV is in a dephosphorylated state (3, 74, 77). Sequence comparison with the available B. anthracis sequence suggests that a gene encoding such a phosphatase is also present in B. anthracis. It has also been suggested that S. aureus contains additional regulators because the synthesis of its sigma B homolog responds to both energy and environmental stress (14). The B. anthracis sigma B homolog also responds to heat shock. However, we have identified no RsbR, RsbS, RsbT, RsbU, or RsbX homolog in the available B. anthracis sequence. Therefore, if other regulators exist, they have little sequence similarity to their B. subtilis homologs.

The recognized role of L. monocytogenes sigma B in osmotolerance led to the suggestion that the role of the B. subtilis sigma B regulon may have diminished partly due to the development of other adaptative responses such as sporulation (7). One of our goals when we began working on the B. anthracis sigB operon was to determine whether it was more similar to those of other Bacillus species or to those of other pathogenic bacteria. In fact, with the absence of rsbX, it seems to be most similar to that of the most distant bacterium, S. aureus, because L. monocytogenes, although nonsporulating, belongs to the Bacillaceae. The stresses encountered by these pathogenic bacteria, one intracellular and the other extracellular, are probably different. Since they enter the host as vegetative cells, the stresses they encounter may also differ from those experienced by B. anthracis. Indeed, the currently accepted life cycle of B. anthracis stipulates that it has no multiplication cycle outside the host and that its infecting form is the highly resistant spore. It was therefore unclear why this bacterium has a general stress regulon. However, our data indicate that the sigma B mutant was less virulent than the parental strain, suggesting that under physiological conditions sigma B may confer an advantage and indicating that sigma B is a minor virulence factor. This may not be the most important contribution of this transcription factor to the persistence of B. anthracis. The last stage of anthrax is septicemia, and the bacilli do not sporulate unless they have access to external oxygen (in outflowing body fluids or if the carcass is opened). These bacteria therefore have to survive as nongrowing vegetative cells, and sigma B may be important at this stage. We therefore suggest that the B. anthracis and B. subtilis sigma B regulons may play similar roles. The stress-resistant state of growth-restricted cells in the mammalian environment for B. anthracis and under certain soil conditions for B. subtilis would constitute the alternative survival mechanism if sporulation was hampered, although the stresses experienced are different (36, 80). Thus, in B. anthracis, sigma B is probably a minor virulence factor and a persistence factor.


    ACKNOWLEDGMENTS

We thank M. Mock, in whose laboratory this work was conducted, for her constant interest. We also thank M. Lévy for the LD50 determination, M. A. Lopez-Vernaza for construction of the GSB2 and GSB12 strains, T. Msadek for providing strains, T. Mignot for critical reading of the manuscript, and A. L. Sonenshein for providing plasmids and for fruitful discussions. TIGR is also acknowledged for making the unfinished Bacillus anthracis sequence data available.

The work at TIGR is funded by ONR/DOE/NIH/DERA. O.N. is a DGA fellow.


    FOOTNOTES

* Corresponding author. Mailing address: Toxines et Pathogénie Bactériennes, Institut Pasteur, 28 rue du Dr Roux, 75724 Paris cedex 15, France. Phone: 33 1 45 68 86 54. Fax: 33 1 45 68 89 54. E-mail: afouet{at}pasteur.fr.

dagger Present address: Institut de Génétique et Microbiologie, CNRS UMR 8621, Laboratoire de Génétique Moléculaire de la Traduction, Université Paris-Sud, 91405 Orsay cedex, France.


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Abstract
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Materials and Methods
Results
Discussion
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Journal of Bacteriology, September 2000, p. 5036-5045, Vol. 182, No. 18
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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