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Journal of Bacteriology, December 1998, p. 6468-6475, Vol. 180, No. 24
Laboratoire de Microbiologie de
l'Université Libre de Bruxelles1 and
Institut de Recherches Microbiologiques J.-M.
Wiame,2 B-1070 Brussels, Belgium
Received 25 June 1998/Accepted 6 October 1998
The arginine deiminase pathway enables Bacillus
licheniformis to grow anaerobically on arginine. Both the
presence of arginine and anaerobiosis are needed to trigger induction
of the pathway. In this study we have cloned and sequenced the
arc genes encoding the pathway. They appear clustered in an
operon-like structure in the order arcA (arginine
deiminase), arcB (ornithine carbamoyltransferase), arcD (putative arginine-ornithine antiporter),
arcC (carbamate kinase). It was found that B. licheniformis has an arginine repressor, ArgR, homologous to the
B. subtilis arginine repressor AhrC. Mutants affected in
argR were isolated. These mutants have lost both repression by arginine of the anabolic ornithine carbamoyltransferase and induction of the arginine deiminase pathway. Electrophoretic band shift
experiments and DNase I footprinting revealed that in the presence of
arginine, ArgR binds to a site upstream from the arc promoter. The binding site is centered 108 nucleotides upstream from
the transcription start point and contains a single Arg box.
Bacillus licheniformis is
a facultative anaerobic bacterium. In the absence of oxygen, it can
derive the energy needed for growth from fermentation of glucose,
respiration of nitrate, or degradation of L-arginine
(7). The arginine deiminase pathway enabling
arginine-dependent anaerobic growth consists of three enzymatic
steps. In the last step, catalyzed by carbamate kinase, the phosphate
group of carbamoylphosphate is used to phosphorylate one ADP to ATP
(see Fig. 1). A second pathway of L-arginine catabolism, the arginase pathway (Fig. 1), enables the cell to use arginine as
a carbon and nitrogen source (46). Although arginine is
required for induction of either pathway, additional regulations
prevent the simultaneous presence of both pathways at high levels
(7): good aeration favors the arginase pathway, while
anaerobic conditions repress it and induce the arginine deiminase
pathway (7). Regulation of the latter has been extensively
studied in the gram-negative bacterium Pseudomonas
aeruginosa, where the pathway is encoded by four genes organized
in an operon: arcDABC (5, 38, 47). Expression of
the arc genes in P. aeruginosa is up-regulated by the Anr protein, an activator belonging to the Fnr family of
transcriptional regulators (20, 21). Anr mediates induction
by anaerobiosis. In contrast to the situation in B. licheniformis, induction of the arginine deiminase pathway does
not require arginine in P. aeruginosa (41).
Regulation of the aerobic arginase pathway in Bacillus
subtilis has been studied. Arginine is a source of nitrogen but
not of carbon for B. subtilis, and arginase is its only
pathway of arginine catabolism. The operons rocABC
(25) and rocDEF (23), which encode
the enzymes and permeases of the pathway, have promoter elements at
Strains and plasmids.
The strains and plasmids used in this
study are listed in Table 1.
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
The arcABDC Gene Cluster, Encoding the Arginine
Deiminase Pathway of Bacillus licheniformis, and Its
Activation by the Arginine Repressor ArgR

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ABSTRACT
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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INTRODUCTION
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
12 and
24, and their transcription is driven by a specialized sigma
factor, a product of the sigL gene, and the B. subtilis homologue of sigma54 (16).
Transcription from the promoters of these operons is dependent on the
product of the rocR gene (9, 22, 23) and on AhrC
(22, 34, 43), initially characterized as the repressor of
the biosynthetic genes (43). AhrC is homologous to the
Escherichia coli arginine repressor ArgR (44).
Several other gram-positive bacteria are reported to possess an ArgR-
or AhrC-like protein (18, 45, 48), and it seemed likely that
there is one in B. licheniformis, since arginine
availability controls the levels of the arginine biosynthetic enzymes
(6). We have recently resumed the study of the B. licheniformis arginine deiminase to understand the mechanism of
dual regulation by arginine and oxygen. Because AhrC has an activator
function in B. subtilis, we tested the hypothesis that
an ArgR- or AhrC-like protein plays a similar role in induction of the
arginine deiminase pathway in B. licheniformis. This led us to characterize the ArgR protein of B. licheniformis and investigate its role in induction of the
arginine deiminase pathway. The arc genes encoding the
pathway were cloned and sequenced. The transcription initiation point
was identified, and the sequences upstream from it were analyzed to see
if some might be involved in regulation.
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MATERIALS AND METHODS
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
TABLE 1.
Strains and plasmids
Cultures and growth conditions. All cultures were grown at 37°C on a rotary shaker. Two culture media were used: Luria-Bertani (LB) broth and minimal medium 154 (57) supplemented with thiamine (1 µg/ml); carbon and nitrogen sources were added to minimal medium to the final concentration of 20 mM, except for arginine, which, in oxygen-limited or anaerobic cultures, was added to the concentration of 40 mM. Two types of culture were used for the enzymatic assays. Well-aerated cultures were obtained by growing the cells in a volume of medium equal to 1/10 of the total volume of the culture flask. Growth was checked by monitoring the optical density at 660 nm, and the cells were harvested in mid-exponential phase. Limited aeration (promoting good induction of the arginine deiminase pathway in the wild type) was achieved as follows. Each strain to be tested was grown as a well-aerated culture on medium 154 plus glutamine to the early exponential phase (3 × 107 cells/ml), and then arginine was added and the culture was poured into a smaller culture flask with a capacity not exceeding 125% of the culture volume. The culture was incubated further for 18 h with shaking. The aerobic start ensured a good cell yield even for mutants impaired in anaerobic growth.
Selection of transformants. Selection of ampicillin-resistant E. coli transformants was done on LB agar plates supplemented with 50 µg of ampicillin per ml. Selection for argF complementation in transformants of the E. coli C600 derivative was done on solid minimal glucose-ammonium medium supplemented with ampicillin (50 µg/ml) and proline (1 mM).
Selection of hydroxamate-resistant mutants. Arginine-hydroxamate-resistant colonies appeared spontaneously when the wild-type strain was streaked on solid 154 medium supplemented with glucose, ammonium, and arginine hydroxamate (200 or 400 mM). The resistance mutants were purified on the same medium. Their capacity to grow anaerobically on arginine was tested on solid 154 medium with 40 mM arginine and 20 mM pyruvate. The plates were incubated in a GasPak jar with Anaerocult A (Merck) reagents to produce the anaerobic environment.
Ornithine carbamoyltransferase assay. Although they function in opposite directions in the cell, both the anabolic and the catabolic ornithine carbamoyltransferases were assayed in vitro for the production of citrulline in the presence of carbamoylphosphate and ornithine. Since they have the same optimum pH, they are distinguishable only by their kinetic behavior (36). The anabolic enzyme has a characteristic saturation curve for ornithine. Its activity peaks at 3 mM ornithine and decreases as the ornithine concentration is further raised, the activity at 50 mM representing only 30% of the activity at 3 mM. The catabolic enzyme displays a typical Michaelis-Menten curve. It is saturated at 8 mM ornithine, and its activity is maintained beyond 50 mM. Ornithine carbamoyltransferase activity was therefore always assayed at two ornithine concentrations (3 and 50 mM) in order to assess whether it was due to the catabolic or the anabolic enzyme. Assays were performed at 37°C according to the method of Broman et al. (6) in a reaction mixture containing 50 mM EDTA-NaOH buffer (pH 8.5), 3 or 50 mM ornithine, 5 mM carbamoylphosphate, and sonicated cell extract. Archibald's colorimetric assay (2) was used to measure the amount of citrulline formed. Protein concentrations were determined by Lowry's procedure.
In what follows, specific activities are expressed in micromoles of product formed per hour and per milligram of protein.DNA manipulations. Restriction reactions, ligation, and cell transformation were done according to standard procedures (50). Plasmid DNA was purified on Nucleobond AX PC100 columns (Macherey-Nagel).
Determination of the transcription start site.
Primer
extension was performed according to the method of
Débarbouillé and Raibaud (15) with an RNeasy kit
from Qiagen, Moloney murine leukemia virus reverse transcriptase, and
50 to 100 µg of total RNA extracted from oxygen-limited cultures
grown on glutamine-arginine medium. Two oligoprimers were used: Adi
P3 (5'-CGTGTATCGGTGTTGTCATGA-3'), which hybridizes with
nucleotides 25 to 5 of the arcA gene, and ArcD-C
13
(5'-CCGATGACAAGCGCGATGAGCGCA-3'), which corresponds to
nucleotides 53 to 30 of the arcD gene.
PCR conditions. PCRs were performed with Taq polymerase in a Techne thermocycler (Progene). A typical procedure consisted of 30 cycles (30 s at 94°C, 30 s at 50°C, and 2 min at 72°C), preceded by 3 min at 94°C and followed by 10 min at 72°C. During inverse PCR, the elongation time was adapted to the length of the fragment to be amplified. The reaction mixture (100 µl) composition was as recommended by the manufacturer of the polymerase (Boehringer Mannheim). Genomic DNA to be used as a template in the PCR was purified on a Qiagen RNA/DNA maxi kit column according to the manufacturer's instructions.
Cloning of a 5' arcA' fragment. The sequence located upstream from the HindIII site and believed to contain the ATG codon and the beginning of the arcA gene was obtained by PCR amplification applied to a genomic library of Sau3A fragments cloned at the BamHI site of plasmid YEp52b. An oligonucleotide (ADI-p, 5'-CGGATCTCTTGTGAAATAAAGGTTCGGC-3') that hybridizes with nucleotides 464 to 492 of the B. licheniformis arcA gene, combined with an oligonucleotide (Leu-2, 5'-ATAATGGTGAAAGTTCCCTCAAGA-3') that matches part of the Saccharomyces cerevisiae LEU2 gene carried by this plasmid and located 570 bp from the BamHI cloning site, was used to amplify a fragment of about 1,300 bp, which was cloned in plasmid pCR2.1.
Cloning of the argR gene.
B. subtilis
and B. licheniformis being closely related, it was
expected that if an AhrC homologue exists in B. licheniformis, it would be very similar to the B. subtilis protein. Therefore, two degenerate oligonucleotides were
designed on the basis of the B. subtilis ahrC
(44) and Bacillus stearothermophilus argR sequences (18): Dahrc+1 (5'-TGAACAAAGGSCARAGGCATATT-3')
and Dahrc
2 (5'-CCGGCARATRATTAAAMWCGTAT-3'). With
this pair of primers, we amplified a 398-bp fragment, which was then
cloned in plasmid pCR2.1 and sequenced. Sequence similarities between
the fragment and the B. subtilis ahrC gene confirmed
that we had indeed cloned part of a homologous gene, which we called
argR. Inverse PCR was used to complete the sequence. Genomic
DNA was restricted with HindIII. The circular fragments
obtained after ligation served as templates for PCR amplification with
a pair of divergent primers, ArgR+4
(5'-CTCAGCCAGCCATCTGATTGTGTT-3') and ArgR
3
(5'-TTCAATTTCATTCGCGGTGATAAT-3'), which match parts of the
B. licheniformis sequence. The 1,300-bp fragment
amplified in this experiment contained the remainder of the
argR sequence. Two primers matching sequences upstream and
downstream from the gene (Nd-argR+5,
5'-GCCATATGAACAAAGGTCAAAGG-3', and Nd-argR
6,
5'-GCCATATGTCAGGCCTTCGTTTA-3') were then used to amplify the
complete gene, which was subsequently cloned in plasmid pCR2.1.
Production and purification of the ArgR protein.
The
NdeI sites present on the Nd-argR+5 and Nd-argR
6
oligoprobes used to amplify the argR gene before cloning
allowed excision of the argR insert and its subcloning into
the NdeI cloning site of the expression vector pET-3a. The
correct orientation of the insert was confirmed by the SmaI
restriction pattern. The recombinant plasmid was stable in E. coli HMS174(DE3)pLysS. For production of ArgR, the strain was
grown in LB broth to mid-exponential phase. Overexpression was then
triggered by addition of IPTG
(isopropyl-
-D-thiogalactopyranoside; 1 mM) and allowed
to proceed for another three hours before cells were harvested. After
sonication of the cells in 10 mM Tris-HCl, pH 8, ArgR was recovered in
the centrifugation pellet and solubilized in the extraction buffer by
addition of NaCl (final concentration, 1.5 M) according to the
procedure used to purify the B. subtilis AhrC protein
(14). The protein appeared highly purified after this single
step, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of
the preparations revealed a single band at the expected
Mr of 16,800 after the gel was stained with
Coomassie brilliant blue. The yield was typically 2 mg of protein for a 100-ml culture. Protein preparations were stored at 4°C.
In vitro binding of ArgR.
A 272-bp fragment corresponding to
nucleotides
221 to +51 with respect to the transcription initiation
point was produced by PCR from genomic DNA; the
oligonucleotides used were Arc+P1 (5'-CAGCTTTTTTTGCCCTTCAA-3')
and Adi
P3 (5'-CGTGTATCGGTGTTGTCATGA-3'). One of the
two oligoprimers (according to the DNA strand to be visualized in the
DNase I footprinting experiments) was labeled at its 5' end with
[
-32P]ATP (Amersham) and T4 polynucleotide kinase
before PCR. The labeled fragment produced by PCR was purified from a
nondenaturing 6% (wt/vol) polyacrylamide gel. Band shift experiments
and DNase I footprinting (19) were performed on this
fragment with purified ArgR protein, as described by Charlier et al.
(10).
DNA sequencing.
All DNA sequences were read on both strands
of plasmid DNA by the enzymatic chain termination method
(51) with a T7 polymerase sequencing kit (Pharmacia) and
[
-35S]dATP (Amersham). When clones resulted from
ligation of a PCR amplification product (as with the wild-type and
mutant argR genes), sequencing was repeated on two different
clones obtained from two independent PCRs. For sequencing of the
HindIII-BamHI arc fragment,
overlapping deletions were created with exonuclease III and S1 nuclease
(50). The antiparallel strand was sequenced with synthetic
oligonucleotide primers designed to hybridize with the first sequence.
Sequence analysis. Database searches were done with the BLAST program (1), and multiple alignments of protein sequences were done with CLUSTAL W (58).
Nucleotide sequence accession numbers. The nucleotide sequences obtained in this work have been deposited with the DDBJ/EMBLGenBank databases under the accession no. Y17554 (arc sequences) and Y17553 (argR).
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RESULTS |
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Cloning of the arcB gene. B. licheniformis possesses two ornithine carbamoyltransferases, one specifically associated with arginine biosynthesis and the other associated with arginine catabolism (6) (Fig. 1). In vitro, both of these enzymes catalyze citrulline synthesis with equal efficiencies (6), since the equilibrium constant of the reaction favors citrulline synthesis (3, 56). Therefore, complementation of an argF mutation in E. coli seemed an appropriate strategy for selecting a clone bearing arcB, the gene encoding the catabolic ornithine carbamoyltransferase. Hence, plasmid pHin4a was selected from a genomic library of HindIII restriction fragments in plasmid pGEM-7Zf for its ability to restore growth of an argF argI derivative of E. coli C600 on arginine-free minimal medium. The enzyme produced from the plasmid clearly displayed the kinetic features of the B. licheniformis catabolic carbamoyltransferase. Plasmid pHin4a was found to contain a 10-kb insert. A restriction map of this HindIII restriction fragment was made, and a 5-kb HindIII-BamHI fragment having retained the ability to complement the argF mutation was subcloned (Fig. 2). The HindIII-BamHI fragment of the resulting plasmid pHin4b was entirely sequenced. Sequence analysis revealed three possible open reading frames, preceded by a truncated one, all transcribed in the same direction. The first complete open reading frame (1,008 bp) nearest the HindIII site was easily identified as the arcB gene, which encodes the catabolic ornithine carbamoyltransferase, as the 5' end of its translated sequence exactly matched the first 23 residues of the N-terminal portion of the purified protein (60a). Furthermore, extensive similarities were found between the B. licheniformis arcB gene product and all other available ornithine carbamoyltransferases, upon alignment of their sequences. The arcB product (335 amino acids, 37.6 kDa) was notably compared with the sequence of the much-studied catabolic ornithine carbamoyltransferase of P. aeruginosa (4) (Fig. 3). The motifs S-T-R-T-R (residues 57 to 61 in P. aeruginosa) and H-P-T-Q (residues 135 to 138 in P. aeruginosa), conserved among carbamoyltransferases (4, 33), are present in the B. licheniformis sequence. Also present are several residues located within and between the S-T-R-T and H-P-T-Q motifs, whose involvement in catalysis or carbamoylphosphate binding has been demonstrated in studies of other carbamoyltransferases: Ser-57, Arg-59, Thr-60, Lys-88, Arg-108, Gly-129, His-135, and Gln-138 (26, 32, 35). The ornithine binding site is present as well, i.e., the Cys-274 residue (27, 40) surrounded by the conserved motif F-M/L-H-C-L-P (residues 271 to 276 in P. aeruginosa).
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Identification of the arcA, arcC, and arcD genes. Homology searches enabled us to locate the arcC and arcD genes on the same HindIII-BamHI restriction fragment (Fig. 2). The 1,407-bp open reading frame immediately downstream from arcB encodes a 50-kDa protein. The high similarity (46% identity) observed between this protein and the arginine-ornithine antiporter of P. aeruginosa (38) suggests that this open reading frame corresponds to the arcD gene. The third complete open reading frame (951 bp) was identified as the arcC gene. It encodes a 316-residue, 33.7-kDa protein 39% identical to the P. aeruginosa carbamate kinase (5). Analysis of the sequences upstream from the arcB gene revealed the existence of a fourth possible open reading frame, truncated at the HindIII site. Translation of the 1,009 bp of this incomplete open reading frame indicated similarity between its product and P. aeruginosa arginine deiminase, the product of the arcA gene (5). We thus used inverse PCR as detailed in Materials and Methods to obtain the sequence upstream from the HindIII site expected to contain the ATG codon and the beginning of the arcA gene. The initial sequence was extended by 478 bp upstream from the HindIII site, and the ATG codon of the arcA gene was found 230 bp upstream from the site. Sequence analysis showed that the arcA gene (1,242 bp) encodes a 413-residue, 47.4-kDa protein showing 34% identity with the arginine deiminase of P. aeruginosa (5).
Homology searches were carried out with the remaining sequences: the 248 bp upstream from arcA and the 321 bp separating the TAA stop codon of arcC from the downstream BamHI site. No additional open reading frames were identified. Thus, similarity to P. aeruginosa genes alone enabled us to identify the four open reading frames of the HindIII-BamHI restriction fragment. Even higher similarities were observed when the B. licheniformis sequences were compared with those of the homologous proteins of gram-positive bacteria: Lactobacillus sake (62), with 58% identity in arcA, 67% in arcB, 51% in arcC, and 43% in arcD, and Clostridium perfringens (45), with 53% identity in arcA, 56% in arcB, 51% in arcD, and 42% in arcC.Analysis of the arc cluster.
Sequencing of
the HindIII-BamHI fragment showed
that the four arc genes encoding the entire arginine
deiminase pathway are clustered on the B. licheniformis
chromosome. They are transcribed in the same direction and separated by
small intergenic spaces: 28 bp between arcA and
arcB, 53 bp between arcB and arcD, and 21 bp between arcD and arcC. A typical ribosome
binding site was recognized upstream from each gene: GAGG upstream from
the ATG codons of arcA and arcD, GGAGAG
(arcB), and AGAGG (arcC). Primer extension
experiments were carried out to locate transcriptional start points in
the cluster. Two different oligoprimers were used: Adi
P3 (which
hybridizes in arcA) and ArcD-C
13 (which hybridizes in
arcD). The choice of an oligonucleotide complementary to
arcD was justified by the somewhat larger intergenic space
and the presence of an inverted repeat at the end of arcB
that might belong to a terminator. No transcription start point was
found in the 200 bp immediately upstream from the arcD gene.
Mapping of the extension products indicated that the adenine located 25 nucleotides upstream from the arcA ATG codon is the
transcriptional starting point (data not shown).
116 to
99 with respect to the transcription start point,
15 of the 18 nucleotides match the E. coli consensus
Arg box (10, 13, 39) (Fig. 2). Similar Arg boxes are found
on the biosynthetic argCJBD-cpa-argF
operons of B. subtilis and B. stearothermophilus (52, 53), where they are the targets
of the arginine repressor.
Downstream from the putative Arg box, in the middle of a zone of dyad
symmetry centered at
65.5, the imperfect palindromic sequence
5'-ACATGTGAAATATATCACGTTT-3' is very close to the Crp consensus sequence of E. coli
(5'-AA-TGTGA--T---TCACA-T-T-3') (24) and to the
putative Fnr binding site of B. subtilis
(5'-A-A-TTGAT--A-ATCAAT----3') (12). The Fnr and
Crp proteins belong to the same family of regulatory proteins and bind
to similar, well-conserved sequences (55). The existence of
two possible binding sites very near the arc promoter
suggests the involvement of at least two regulatory proteins, one of
the ArgR or AhrC type and one Fnr family member.
Sequence analysis of the argR gene. The strategy used to clone the B. licheniformis argR gene was based on the assumed high homology between the arginine repressors of B. licheniformis and B. subtilis. This homology was confirmed by alignment of the complete sequences: ArgR is 89% identical to its B. subtilis homologue (44) and 71% identical to its B. stearothermophilus homologue (18). Other gram-positive ArgR proteins are much less similar, the percentages of identical residues being in the same range as for the proteins of gram-negative bacteria; the B. licheniformis protein is 34% identical to Mycobacterium tuberculosis ArgR (accession no. Z85982), 31% identical to the C. perfringens protein (45), and 32% identical to the E. coli protein (37).
The B. licheniformis arginine regulator contains both highly conserved domains already revealed by aligning the B. subtilis (44) and B. stearothermophilus (18) sequences with the E. coli sequence (37) (Fig. 4). One domain, located in the N-terminal part of the protein, is the DNA binding site (28), which contains, notably, the SR motif (residues 47 and 48 in E. coli) involved in DNA binding (60). The second domain, in the C-terminal region, is the arginine-binding domain, defined by successive studies of E. coli (8, 60, 61). In the E. coli protein, this domain spans residues 80 to 156 and contains residues A105, D128, and D129, the substitution of which affects arginine binding (8, 60), and G123, involved in oligomerization (11, 60). All the corresponding amino acids are conserved in both B. licheniformis and B. subtilis.
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Characterization of the ArgR protein. The 447 bp of the argR gene encodes a 149-residue polypeptide chain with a calculated molecular mass of 16.82 kDa. Purified ArgR protein electrophoresed under native conditions revealed a major band at 100 kDa. This result is consistent with a hexameric structure, as for the B. subtilis (14) and E. coli (37) arginine repressors. A very faint band at 50 kDa may be due to the presence of the trimeric form in small proportion.
ArgR binds in vitro to a region upstream from the arc
promoter.
By producing and purifying the B. licheniformis ArgR protein we were able to test whether its target
is indeed the putative Arg box upstream from the arc
promoter. Band shift assays clearly showed that purified ArgR binds to
a 272-bp DNA fragment bearing the arc promoter
(nucleotides
221 to +51 with respect to the transcription start
site), which reduces its electrophoretic mobility, and that this
binding is arginine dependent (Fig. 5).
In footprinting experiments with the same 272-bp fragment, binding
of ArgR protected a single stretch of 27 nucleotides on each strand
against DNase I digestion (Fig. 2 and 6).
There was only a 19-bp overlap, however, between the regions
protected on the two strands. This stretch, centered at
108, contains
the putative Arg box revealed by sequence analysis. While the limits of
the Arg box and of the protected region differ by no more than 1 nucleotide at the 5' end, protection is extended by 8 or 9 nucleotides
beyond the limit of the Arg box towards the 3' end. The difference is
probably too large to be solely attributed to the recognition and
cutting mode of DNase I in the minor groove of the DNA helix. Similar
differences have been observed at other ArgR binding sites (18,
34). The length of the protected region (27 bp) is markedly
smaller than the region protected by ArgR in the different
E. coli operators (on average, 40 bp), where two Arg
boxes are present (10, 59). In the operators of the
B. subtilis genes involved in arginine catabolism, only one Arg box has been identified and AhrC again protects smaller regions: 17 nucleotides in OrocD and 19 to
21 nucleotides in OrocA (42).
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Induction of the arginine deiminase pathway does not occur in argR mutants. In B. subtilis, mutations in the gene coding for the AhrC arginine repressor confer resistance to arginine hydroxamate, an arginine analogue. In ahrC mutants, the arginine-biosynthetic enzymes are present at high, constitutive levels and their synthesis is not repressed in the presence of exogenous arginine (29, 43). Arginine hydroxamate is also a growth inhibitor of B. licheniformis. Spontaneously arising mutants displaying resistance to arginine hydroxamate were selected on analogue-supplemented minimal glucose-ammonium medium. Five such mutants (Ahr200-6, Ahr400-6, Ahr400-7, AhrO-3, and AhrO-7) had a phenotype consistent with an argR or ahrC mutation, showing high constitutive ornithine carbamoyltransferase activity on well-aerated glucose-ammonium medium (Table 2). The ratio of the activity measured at 50 mM ornithine to that measured at 3 mM ornithine confirmed that the activity was due to the anabolic ornithine carbamoyltransferase. Although some mutants (Ahr200-6, Ahr400-6, and Ahr400-7) displayed a somewhat different activity when arginine was added to the glucose-ammonium medium, this was probably not significant, as the standard deviation of such high activities can represent up to 23% of the specific activity measured (Table 2). All constitutive mutants were impaired in their ability to grow anaerobically on arginine-pyruvate medium, apparently as a consequence of their inability to induce the arginine deiminase pathway (Table 2). During oxygen-limited growth, they displayed low specific ornithine carbamoyltransferase activity, resulting from the presence of residual catabolic and anabolic enzyme.
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DISCUSSION |
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By cloning and sequencing the arc genes encoding the arginine deiminase pathway of Bacillus licheniformis, we have shown that these genes are clustered on the genome of this species. The arc genes appear in the same order as in another gram-positive bacterium, C. perfringens (45): arcA-arcB-arcD-arcC. A different order, (arcD)-arcA-arcB-arcC, is found in the gram-negative bacteria P. aeruginosa (4, 5, 38) and Rhizobium etli (17). Mycoplasma pneumoniae (31) and Halobacterium salinarium (49) also display a different organization of the arc genes.
In B. licheniformis, a protein homologous to the arginine regulator AhrC of B. subtilis acts as a repressor of the arginine-biosynthetic pathway. The DNase I footprinting data show that the purified protein, called ArgR, binds to a single Arg box, revealed by analysis of the long noncoding sequence preceding arcA. We have isolated five argR mutants by selection for resistance to the arginine analogue arginine hydroxamate. Their phenotype confirms that in the presence of arginine, B. licheniformis ArgR is both a repressor of the anabolic ornithine carbamoyltransferase and an activator of the arginine deiminase pathway. The inability of the argR mutants to induce the arginine deiminase pathway after oxygen depletion strongly suggests that induction of the arginine deiminase pathway by arginine requires binding of the ArgR protein to the Arg box located upstream from the arc promoter.
Several differences between activation of the arc genes by
ArgR in B. licheniformis and activation of the
rocABC and rocDEF operons by AhrC in
B. subtilis are apparent. In the roc
operons, the region recognized by AhrC is immediately adjacent to the
transcription start point (34, 42), while in B. licheniformis, ArgR binds 108 bp upstream from the arc
transcription start point. Moreover, the promoters of the
roc operons are of the
12/
24 type; their transcription
requires a specific sigma factor, a product of the sigL gene
(16), and binding of the RocR activator to two UAS sequences
(9, 22, 23). It has been suggested that activation results
from DNA bending induced by AhrC binding (34, 42), but on
the other hand the recent evidence presented by Gardan et al.
(22) suggests that interaction between the AhrC and RocR proteins directly causes activation. The arc promoter of
B. licheniformis bears no resemblance to the
12/
24
roc promoters, and there are no stretches resembling the
Bacillus
A-dependent consensus promoter
sequence at
10 and
35 (30). The Bacillus
licheniformis promoter does contain, between the ArgR binding site
and the site covered by RNA polymerase, an imperfect palindromic
sequence similar to the E. coli Crp and B. subtilis Fnr consensus sequences. This finding suggests the
involvement of a second regulatory protein, possibly belonging to the
Crp or Fnr family. Since many Crp or Fnr family members are involved in
anaerobic regulations (54), it is tempting to hypothesize that the second regulatory protein is the anaerobic activator of the system.
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ACKNOWLEDGMENTS |
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This work was supported by the Fonds pour l'Encouragement de la Recherche (Université Libre de Bruxelles) and the Belgian Fund for Joint Basic Research.
We thank J.-P. Ten Have for his skillful assistance with the figures.
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FOOTNOTES |
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* Corresponding author. Mailing address: Institut de Recherches Microbiologiques J.-M. Wiame, Ave. E. Gryson, 1, B-1070 Brussels, Belgium. Phone: 322 526 7272. Fax: 322 526 7273. E-mail: ceriair{at}ulb.ac.be.
Present address: av. dos Bombeiros, 42, 2765 Estoril, Portugal.
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