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Journal of Bacteriology, August 1998, p. 4219-4226, Vol. 180, No. 16
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Phosphate Assimilation in Rhizobium (Sinorhizobium) meliloti: Identification of a pit-Like Gene

Sylvie D. Bardin, Ralf T. Voegele, and Turlough M. Finan*

Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1

Received 10 September 1997/Accepted 1 June 1998

    ABSTRACT
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

Rhizobium meliloti mutants defective in the phoCDET-encoded phosphate transport system form root nodules on alfalfa plants that fail to fix nitrogen (Fix-). We have previously reported that two classes of second-site mutations can suppress the Fix- phenotype of phoCDET mutants to Fix+. Here we show that one of these suppressor loci (sfx1) contains two genes, orfA and pit, which appear to form an operon transcribed in the order orfA-pit. The Pit protein is homologous to various phosphate transporters, and we present evidence that three suppressor mutations arose from a single thymidine deletion in a hepta-thymidine sequence centered 54 nucleotides upstream of the orfA transcription start site. This mutation increased the level of orfA-pit transcription. These data, together with previous biochemical evidence, show that the orfA-pit genes encode a Pi transport system that is expressed in wild-type cells grown with excess Pi but repressed in cells under conditions of Pi limitation. In phoCDET mutant cells, orfA-pit expression is repressed, but this repression is alleviated by the second-site suppressor mutations. Suppression increases orfA-pit expression compensating for the deficiencies in phosphate assimilation and symbiosis of the phoCDET mutants.

    INTRODUCTION
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

Because phosphate readily forms insoluble mineral phosphate complexes with other ions found in soil, the concentrations of soluble or available phosphate detected in soil solutions are low and generally range from 0.1 to 10 µM (3). While many Rhizobium strains are able to grow at these low phosphate concentrations (6), phosphate limitation often reduces dinitrogen fixation of legume-Rhizobium interactions by reducing both nodule number and mass (18, 26, 28). In addition, phosphate limitation may directly affect discrete steps during the nodule formation and infection process, such as excretion of Nod factors (22) and/or attachment to roots (17). Additional bacterial phenotypes of symbiotic importance which change in response to phosphate limitation include the biosynthesis and/or alteration of rhizobial cell surface carbohydrates such as exopolysaccharides II (45) and cyclic beta -(1,2)-glucans in Rhizobium meliloti (5) and lipopolysaccharides in Rhizobium leguminosarum (32).

Our interest in phosphate metabolism in R. meliloti arose through our genetic analysis of the ndvF locus which is located on the pExo megaplasmid. The ndvF mutants form nodules on alfalfa that contain few bacteria and fail to fix N2 (Fix-) (8, 10). Two genetically distinct classes (I and II) of second-site mutations (sfx) which suppressed the ndvF Fix- phenotype were identified (25), and an analysis of those mutations together with a characterization of the ndvF locus has been the subject of several recent reports (1, 2, 37). The ndvF locus was shown to contain four genes, phoCDET, that encode an ABC-type high-affinity system for the uptake of phosphate and possibly phosphonates into the cell (2, 37). The phoCDET mutants were found to grow poorly in minimal media containing 2 mM orthophosphate (Pi) as a sole source of phosphorus (2). Class I suppressor mutations (sfx1, sfx4, and sfx5) are tightly linked in transduction, and we previously reported the localization of the sfx1 locus to an 18-kb BamHI fragment in the cosmid clone pTH56 (25). Here we show that sfx1 possesses two genes, orfA and pit, and that the deduced Pit protein is homologous with phosphate transporters such as the Pit protein of Escherichia coli.

Analysis of class II suppressor mutations revealed that these mapped to the phoU and phoB regulatory genes and that disruption of these genes suppressed the Fix- and Pi growth phenotype of phoCDET (ndvF) mutants (1). We showed that while the phoB gene was required for expression of the phoCDET genes, phoB appeared to play a negative role in regulating expression of the orfA-pit genes. Here we demonstrate that the sfx1 mutation is a promoter-up mutation which increases expression of the orfA-pit operon and allows Pi uptake via the OrfA-Pit system in a phoCDET mutant background. We conclude that this increased Pi uptake is responsible for the symbiotic phenotype associated with phoCDET mutations.

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

Bacterial strains, plasmids, media, and growth conditions. The strains and plasmids used in this work are listed in Table 1. Transcriptional lacZ fusions to pit were made by subcloning the 2.1-kb EcoRI fragments from pTH354 (wild type) and pTH380 (sfx1) which included the recF-orfA intergenic region into the EcoRI site of the reporter plasmid pMP220 (31) to create plasmids pTH376 and pTH365, respectively. Transcriptional lacZ fusions to orfA were made by deleting the region between the SphI site located in the orfA gene and the SphI site of the pMP220 polylinker of plasmids pTH376 and pTH365 to create plasmids pTH378 and pTH367, respectively.

                              
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  		TABLE 1.   Bacterial strains and plasmids

Plasmids pTH396 and pTH397 were constructed to delineate the fragment in which the sfx1 mutation was located. Since the mutation was suspected to be in the promoter region, the 920-bp XhoI/EcoRI3 and 540-bp EcoRV1/EcoRI3 fragments (EcoRI3, for example, represents the EcoRI at the third site on the plasmid map; see Fig. 3) of pTH191 were subcloned as XhoI/HindIII and EcoRV/HindIII restriction sites of pBR322, respectively. The pBR322 vector was chosen because it is mobilizable by pRK600 but cannot replicate in R. meliloti. To provide a selectable marker to identify R. meliloti recombinants, the 3.4-kb HindIII fragment of pGS220 containing the Kmr gene of Tn5 (12) was subcloned into the HindIII site of the two subclones.

The growth media used were Luria-Bertani medium (LB) and LB supplemented with 2.5 mM MgSO4 and 2.5 mM CaCl2 (LBmc) with antibiotic concentrations as previously described (8). The phosphate-free medium was the MOPS (morpholinepropanesulfonic acid)-buffered minimal medium described by Bardin et al. (1, 2) except that this medium was supplemented with a yeast extract fraction which stimulates growth of R. meliloti in defined medium (14 µl/liter) (41). For the growth experiments, strains grown for 24 h in LBmc were centrifuged and resuspended in phosphate-free MOPS medium (MOPS P0). Twenty microliters of the cells was used to inoculate 5 ml of MOPS P0 (optical density at 600 nm [OD600], ~0.05), and these cultures were grown for 24 h under agitation. This phosphate starvation step was necessary since, otherwise, significant growth occurred in MOPS P0 medium. The OD600 of the cultures was adjusted to 0.2, and 5-µl aliquots were used to inoculate 5 ml of MOPS P0 and 5 ml of MOPS supplemented with 2 mM Pi (MOPS P2). For the beta -galactosidase assays, the LBmc-grown cells, supplemented with tetracycline at 2 µg/ml for the strains containing the pMP220-derived plasmids, were washed once with MOPS P0 and resuspended in this medium. Five-milliliter volumes of MOPS P0 and MOPS P2 were inoculated with 20 (OD600, ~0.05) and 5 (OD600 ~0.015) µl of cells, respectively, and the cells were grown for 38 h before the assay was performed.

DNA manipulation and genetic techniques. Cloning procedures, including DNA isolation, restriction digestions, ligation, and transformation, were performed as described by Sambrook et al. (29). We cloned the wild-type orfA-pit region as a 4.8-kb HindIII-SacI fragment from Rm1021 DNA into plasmid pUC118. One such plasmid, pTH354, was identified via colony hybridization by using a formamide-based hybridization solution at 42°C (described in reference 8) and a digoxigenin-labeled 4.8-kb HindIII-SacI fragment from pTH90 as a probe. Annealed probe was detected with anti-digoxigenin antibody conjugated to alkaline phosphatase (AP).

Conjugal mating with MT616 as the helper strain, Tn5 mutagenesis, and homogenizations (with pPH1JI) were performed as previously described (9, 13, 44).

DNA sequencing and sequence analysis. DNA sequencing of overlapping deletions was performed on single-stranded DNA by using the dideoxy chain termination method as described in the U.S. Biochemicals protocol for the Sequenase 2.0 enzyme and by using [alpha -35S]ATP (NEN DuPont). Single-stranded DNA was obtained from host strain XL-1 Blue (Stratagene) following infection with the helper phage M13K07 (36). Both DNA strands were sequenced by using the -20 (lacZ) primer (5'-GTAAAACGACGGCCAGT-3') and the IS50 primer (5'-TCACATGGAAGTCAGATCCT-3'). The recF-orfA intergenic region was amplified via the PCR by using primers 41, 5'-AAGTCGCTCAGTTTCAGGCGTGTGAGA-3', and 55, 5'-CCCATGACGGTGCGCGAATGATCGG-3' (see Fig. 4). Following gel purification, the sequences of the amplified 359- and 360-bp fragments were determined on an ABI automated DNA sequencer by using the above-described primer 41.

AP and beta -galactosidase assays. The AP activity was measured as described by Yarosh et al. (44) except that the cells were centrifuged before the OD420 was measured. The AP activity was calculated by using the following formula: (1,000 × OD420)/(OD600 × Delta T), with Delta T being the reaction time (in minutes).

The beta -galactosidase assay was performed, and specific activities in Miller units were calculated as previously described (1).

Determination of transcriptional start sites. Total RNA was isolated from MOPS P2 medium-grown cells by using the procedure outlined in the RNeasy Midiprep kit (Qiagen). Three different primers (primers 55, 81, and 82 [see Fig. 4]) were labeled with [gamma -33P]ATP (NEN Dupont) and polynucleotide kinase (NEB). Thirty micrograms of RNA was annealed to labeled primer (4 × 105 to 1.5 × 106 cpm) and extended with Moloney murine leukemia virus reverse transcriptase for 90 min at 42°C. To align the transcriptional start sites, the same primers were used in conjunction with template DNA (pTH191, sfx1) in a standard sequencing reaction by using the Sequenase kit (Amersham, Oakville, Ontario, Canada).

Transport assays. For transport assays, cells were precultured in LBmc, washed three times with MOPS P0, and subcultured into MOPS P2. Cells grown aerobically for 24 h at 30°C were harvested by centrifugation, washed four times with MOPS P0, and resuspended to an OD600 of 10 in MOPS P0. Cells were diluted 1:20 into MOPS P0 and equilibrated for 5 min at 30°C. Uptake was initiated by the addition of [33P]orthophosphoric acid (NEN DuPont) to a final concentration of 4 µM (175 Ci/mol). Aliquots were removed from the assay at different time points, placed on Millipore HAWP 02500 nitrocellulose filters (0.45-µm pore size; presoaked in 1 M K2HPO4-KH2PO4 [pH 7.0]), and immediately washed with 40 mM MOPS-20 mM KOH-20 mM NH4Cl-100 mM NaCl-1.2 mM CaCl2-2 mM MgSO4. Filters were dried, placed in scintillation liquid (BCS; Amersham), and counted. All transport assays were performed in triplicate.

    RESULTS
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

sfx1 suppresses the Pi transport and growth phenotypes of phoCDET mutants. Subsequent to the identification of sfx1 as a second-site mutation which suppressed the Fix- symbiotic phenotype of ndvF mutants (25), we established that the ndvF locus contains four genes, phoCDET, which encode a phosphate transport system (2). Since phoCDET mutants grow slowly in medium containing 2 mM Pi and are defective in Pi transport (2), we wished to determine whether sfx1 restored a wild-type growth and phosphate transport phenotype to phoCDET mutants.

Results from growth experiments showed that while the R. meliloti phoCOmega 490 mutant, RmG490, grew slowly relative to the wild type, Rm1021, in medium containing 2 mM Pi, the doubling time for the phoCOmega 490 sfx1 strain, RmG762, was similar to that of the wild type (Fig. 1). Additional experiments revealed that sfx1 also suppressed the slow-growth phenotype observed when either the phoCDET deletion (Delta G439) mutants or the phoTOmega 491 insertion mutants were grown in MOPS-2 mM Pi (data not shown).


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  		FIG. 1.   Growth of Rm1021 (wild type) (), RmG490 (phoCOmega 490) (bullet ), RmG762 (phoCOmega 490 sfx1) (star ), RmG822 (phoCOmega 490 sfx1 orfA23::Tn5) (black-lozenge ), RmG830 (phoCOmega 490 sfx1 pit10::Tn5) (black-down-triangle ), and RmH842 (phoCOmega 490 sfx1 recF12::TnphoA) (down-triangle) in MOPS-buffered minimal medium supplemented with 2 mM Pi. Each time point represent the average of triplicate values. The growth curve for strain RmG821 [phoCOmega 490 sfx1 (orf2)2::Tn5] was similar to that for strain Rm1021, and the growth curves for phoCOmega 490 sfx1::Tn5 recombinants Omega 16, -3, -J, -5, and -10A (Fig. 3) were similar to those for RmG822 and RmG830. In order to simplify the figure, these results are not shown.

Measurements of the transport of 33P-labeled inorganic phosphate into cells revealed that the rate of Pi uptake in the phoC mutant RmG439 was reduced relative to that of the wild type, Rm1021, whereas uptake into the phoC sfx1 mutant strain, RmG762, was restored to the wild-type level (Fig. 2). Moreover, strain RmG830, which carries the phoCOmega 490 mutation together with a Tn5 insertion in the sfx1 locus (pit10::Tn5), showed Pi uptake rates similar to those of the phoC mutant RmG490. These results established that the sfx1 mutation restored wild-type phosphate transport and growth phenotypes to the phoC mutant. To further investigate the genetic nature of the sfx1 locus, we first delineated the minimal gene region required for sfx1-mediated suppression.


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  		FIG. 2.   Pi uptake in different R. meliloti mutants. [33P]orthophosphoric acid was added to a final concentration of 4 µM. Results for R. meliloti Rm1021 (wild type) (black-lozenge ), RmG490 (phoCOmega 490) (black-triangle), RmG762 (phoCOmega 490 sfx1) (), and RmG830 (phoCOmega 490 pit10::Tn5) (bullet ) are shown. The symbols represent the means of triplicate assays, and each line gives the linear regression for all data points for one strain. Background values for all strains were adjusted so that the results for the extrapolated zero time showed no Pi uptake.

Localization and nucleotide sequence of sfx1. The sfx1 locus was previously located to the 18-kb BamHI fragment on the pTH56 cosmid (25), and as a first step in the delineation of sfx1, a 12-kb HindIII fragment internal to the 18-kb region was subcloned into pRK7813 to give pTH90. This fragment contained the entire sfx1 locus since RmG490 (phoCOmega 490) pTH90 transconjugants formed Fix+ nodules on alfalfa and grew like the wild-type strain in medium containing 2 mM phosphate (data not shown). Tn5 insertions which mapped within the 12-kb insert of pTH90 were isolated, and seven of these plasmids were used to generate pTH90 deletion derivatives in which various amounts of the 12-kb insert region had been removed (Fig. 3, upper diagram). Fragments from the 12-kb region were also subcloned into pRK7813 (Fig. 3). The symbiotic Fix phenotype and/or phosphate growth phenotype of RmG490 (phoCOmega 490) transconjugants carrying the various plasmids allowed us to deduce that the entire sfx1 locus was located between the EcoRI1 and EcoRV1 sites indicated on the map of pTH276 in Fig. 3.


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  		FIG. 3.   Restriction maps of pTH90 and related plasmids. The upper diagram shows the regions which remained following deletion from the BamHI site within Tn5 transposons (Omega 2.15, -1.4, -1.5, -E, -2.3, -3.10, and -2.2) to the BamHI site in the polylinker of pTH90. Below the map for pTH276 are diagrammed the deduced orfA-pit genes and two partial recF and orf2 open reading frames together with the Tn5 insertions that disrupted the sfx1 locus (Omega 16, -3, -J, -5, -10, -23, and -10A) and the Omega 2 and Omega 12 insertions that did not. The orientation of fragments subcloned into pRK7813 are indicated relative to the position of the plac promoter. The subclones are pTH276 (4.8-kb HindIII/SacI), pTH305 (2.6-kb HindIII/EcoRV1), pTH304 (2.5-kb HindIII/SmaI1), and pTH347 and pTH348 (2.6-kb partial EcoRI in orientations I and II). The ability of the plasmids to suppress the Fix- and/or slow-growth phenotype of RmG490 (phoCOmega 490) in MOPS P2 is indicated in parentheses (+, suppression; -, no suppression). Restriction site abbreviations: B, BamHI; Bg, BglII; C, ClaI; H, HindIII; R, EcoRI; Sc, SacI; Sm, SmaI; Sp, SphI; V, EcoRV; X, XhoI. The subscripts following the R and V are used to specify particular sites.

Transposon insertions within this region were recombined into the RmG762 (phoCOmega 490 sfx1) chromosome (Fig. 3), and the physical structure of the resulting recombinants was confirmed by Southern blot analysis (data not shown). The Omega 2 and Omega 12 recombinants grew like the suppressor strain RmG762 in MOPS medium containing 2 mM Pi, whereas the Omega 16, -3, -J, -5, -10, -23, and -10A recombinants grew poorly (data not shown and Fig. 1). In addition, Omega 10 and Omega 23 recombinants formed Fix- nodules when tested on alfalfa plants (data not shown). These results confirmed that sfx1 was located between the Omega 2::Tn5 insertion and the EcoRI3 site. The nucleotide sequence of this 2,828-bp region was determined.

Analysis of the nucleotide sequence of the sfx1 locus. Analysis of the nucleotide sequence revealed the presence of two complete open reading frames designated orfA and pit (see below) and (Fig. 3). The orfA stop codon (TGA) and the pit start codon (ATG) overlapped at the adenosine, suggesting that both genes are transcribed as a single transcript. The G+C contents of orfA and pit were 62 and 64%, respectively. There was a strong bias for G and C at the third position of the codons (83% for orfA and 87% for pit). This bias together with the codon usage of orfA and pit suggested that these genes were expressed in R. meliloti (24, 38).

A partial open reading frame initiating 234 bp upstream from orfA and whose 176-amino-acid product is homologous to the N-terminal regions of many RecF proteins, including Caulobacter crescentus (27), was detected (Fig. 3). An additional partial open reading frame (designated orf2), initiating 114 bp downstream of pit, was also found. Neither recF nor orf2 was part of the sfx1 locus since recFOmega 12::Tn5 and orf2Omega 2::Tn5 recombinants of strain RmG762 (phoCOmega 490 sfx1) retained the suppressor phenotype (see above). An ATG start codon preceded by a potential ribosome binding site was found for each gene (CGGA-N6-ATG for recF, TGGA-N6-ATG for orfA, GAGA-N7-ATG for pit, and GAGA-N7-ATG for orf2).

GenBank searches (16) revealed that the 334-amino-acid R. meliloti Pit protein (RmPit) was similar in sequence to a large family of prokaryotic and eukaryotic phosphate transport proteins (42, 43; for a review, see reference 20), including the low-affinity Pit phosphate transport protein of E. coli (accession no. P37308; EcPitA) (30) and the phosphate-repressible phosphate permease of Neurospora crassa (accession no. P15710; NcPho-4+) (21). In addition, RmPit showed similarity to numerous uncharacterized proteins, including the HI1604 protein of Haemophilus influenza (accession no. P45268) (15) and the EcPitB protein of E. coli (accession no. P43676) (4). Consistent with its homology to Pi transport proteins, an analysis of the RmPit sequence employing the TopPred II program (11) revealed nine "certain" membrane-spanning domains.

GenBank searches revealed that the 214-amino-acid OrfA protein had 21% amino acid identity with the deduced 226-amino-acid HI1603 protein of H. influenzae (accession no. P44271) (15). This result was interesting since the HI1603 gene is located directly upstream (25 bp) from the pit-like HI1604 gene of H. influenzae (see above). The conserved location of orfA-pit-like genes suggests that they are functionally related.

The sfx1 mutation contains a single T deletion in a hepta-T sequence upstream of orfA. To investigate whether the sfx1 mutation lay in the orfA-recF intergenic region, the XhoI/EcoRI3 and EcoRV1/EcoRI3 fragments from the sfx1 locus (Fig. 3) in plasmids pTH396 and pTH397, respectively, were recombined via a Campbell-type single crossover into the RmG490 (phoCOmega 490) genome (see Materials and Methods). The presence of the sfx1 mutation was then determined by screening the recombinants for suppression of the mucoid colony phenotype of phoCDET mutants as observed on low-osmolarity GYM medium (25). Of 70 pTH396 recombinants examined, 12 formed dry colonies and thus suppressed the mucoid phenotype of RmG490. On the other hand, of 70 pTH397 recombinants examined, none showed a suppressor phenotype. Together, these data indicated that the sfx1 mutation lay in the orfA-recF intergenic region.

The nucleotide sequence of the XhoI/EcoRV1 fragments from the sfx1 locus in pTH380 and the wild-type locus in pTH354 were determined. Both sequences were identical, except that a run of seven thymidines, present in the wild-type sequence at positions -80 to -86 upstream from orfA, was reduced to six thymidines in the sfx1 mutant sequence (Fig. 4). A 359-bp fragment containing the complete recF-orfA intergenic region was PCR amplified from the wild type, Rm1021, and the two other class I suppressor strains, RmG203 (sfx4) and RmG204 (sfx5) (25). The nucleotide sequences of the amplified fragments revealed that all three were identical except that sfx4 and sfx5 mutant sequences, like the sfx1 sequence, contained six thymidine residues in place of the seven thymidines present in the wild-type sequence. Thus, all three independently isolated class I suppressor mutants (sfx1, sfx4, and sfx5) carried the same thymidine deletion mutation. While this finding was surprising, we note that repeat sequences, such as the run of seven thymidines, are known to be mutational hot spots (23).


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  		FIG. 4.   Determination of the orfA transcriptional start sites. The autoradiograph shows the products of the sequencing reaction by using primer 55 and plasmid pTH191 (sfx1) as a template. The results for primers 81 and 82 yielded a start site identical to that with primer 55 (data not shown). The relevant sequence is shown on the left, and the arrow indicates the position of the extension product. Lane 1, RmG591 (sfx1) carrying pTH347 (sfx1 orfA-pit in orientation I); lane 2, RmG591; lane 3, Rm1021 (wild type); lane 4, Rm1021 carrying pTH391 (orfA-pit); lane 5, RmG591 carrying pTH348 (sfx1 orfA-pit in orientation II). The sequence on the right of the figure is the wild-type recF-orfA intergenic region. The deduced orfA transcriptional start site is in boldface type and underlined. The translational start sites for orfA and recF are in boldface type with arrows above indicating the direction of transcription. The three different primers used are indicated by thin underlines. The hepta-thymidine repeat is underlined, and a consensus PHO box sequence is shown below the putative PhoB-binding site. Numbers to the right of the sequence are nucleotide positions relative to the orfA transcriptional start site.

sfx1 increases orfA and pit expression. To investigate whether the sfx1 mutation altered the level of orfA-pit expression, we constructed transcriptional lacZ fusions to the orfA and pit genes from both the wild-type and sfx1 loci in the broad-host-range reporter plasmid pMP220 (see Materials and Methods). The resulting plasmids were transferred into the wild-type Lac- R. meliloti strain, RmG212, and the beta -galactosidase (Fig. 5b) and AP (Fig. 5a) activities of the transconjugants were measured after 38 h of growth in MOPS-buffered medium containing either no phosphate (MOPS P0) or 2 mM inorganic phosphate (MOPS P2). High and low AP activities confirmed that the cells were grown under phosphate-deficient and phosphate-sufficient conditions, respectively (Fig. 5a, data sets 1, 2, and 4).


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  		FIG. 5.   Histogram showing AP (a) and beta -galactosidase (b) activities from the R. meliloti wild type (wt), RmG212, and the RmG212 phoC mutant, RmH667. The strains represented in data sets 2 to 5 carried plasmid-borne lacZ gene fusions to the pit or orfA genes from the wild-type and sfx1 loci, i.e., plasmids pTH376, pTH378, pTH365, and pTH367, respectively. Cells were assayed following 38 h of growth in MOPS-buffered minimal medium with no phosphate added () or supplemented with 2 mM phosphate (). Each datum point represents the average of triplicate values ± standard error (error bar).

The results revealed, first, that both pit and orfA expression levels from the sfx1 locus were three and five times higher, respectively, than expression from the wild-type locus (Fig. 5b, data sets 2 and 4). Second, orfA and pit expression was higher in cells cultured with excess Pi (MOPS P2) than in cells cultured with phosphate limitation (MOPS P0). Third, the level of orfA expression was three- to fivefold higher than the level of pit expression. In summary, the sfx1 mutation increased the level of orfA-pit expression and both sfx1- and wild-type-directed expression appeared to be controlled via the phosphate concentration in the medium.

Since the sfx1 mutation exerts its phenotypic effect in a phoCDET (ndvF) mutant background, we also measured both orfA and pit expression in a phoC Lac- background (RmH667). In this background, levels of both orfA and pit expression directed from the wild-type locus were very low regardless of whether the cells were cultured in the presence (2 mM) or absence of inorganic phosphate (Fig. 5b, data sets 3 and 5). On the other hand, while the expression of sfx1-orfA and sfx1-pit gene fusions was reduced by 23 to 30% in the phoC mutant relative to that in a wild-type background, their levels of expression were still two to three times higher than the levels of the corresponding wild-type fusions in a wild-type background. A two- to threefold increase in orfA-pit expression thus appears to be sufficient to allow suppression of the phoCDET phenotypes. We note that the high AP activity detected in phoC mutant cells cultured in medium containing 0 or 2 mM Pi suggests that these cells are starved for Pi (Fig. 5a, data sets 3 and 5) (see also reference 1).

The modest increase in orfA-pit transcription which resulted from the sfx1 mutation prompted us to determine whether an increase in the copy number of the wild-type orfA-pit locus would be sufficient to allow the phoC mutant to grow in medium containing 2 mM Pi. The wild-type and sfx1 orfA-pit gene regions were therefore cloned into the broad-host-range vector pRK7813, and RmG490 (phoC) transconjugants carrying these plasmids (pTH391 and pTH348, respectively) grew like the wild-type strain, Rm1021, in MOPS P2 (data not shown). We therefore conclude that the increase in orfA-pit expression resulting from the increase in the copy number of the wild-type locus is sufficient to suppress the slow growth of the phoC mutant in medium containing 2 mM Pi.

The orfA-pit transcriptional start site lies 29 bp upstream of orfA. To investigate the relationship between the sfx1 mutation and the orfA-pit promoter, we mapped the orfA transcriptional start site by primer extension analysis. mRNA was isolated from strains in which the copy numbers of the sfx1 and wild-type orfA-pit regions were increased via the plasmids pTH391 and pTH348, respectively. A single transcriptional start site, lying 29 bp upstream from orfA, was identified with three separate primers (Fig. 4 and data not shown). The extension product observed from cells carrying additional copies of the wild-type orfA-pit gene region was much less intense than the product from cells carrying additional copies of the sfx1 locus (Fig. 4, lanes 1, 4, and 5). Moreover, in strains carrying only the chromosomal copy of the orfA-pit gene region, a faint extension product was observed with mRNA isolated from sfx1 strains while no product was detected in wild-type cells (Fig. 4, lanes 2 and 3). These results confirmed that the sfx1 mutation increased orfA transcription, and the data also suggest that under growth conditions (2 mM Pi) in which the wild-type orfA-pit operon is maximally expressed, the actual level of transcription is low. These conclusions are consistent with the results from lacZ gene fusion studies described above.

We identified -10 and -35 consensus-like hexamers (TAT GAC and TTTCCC, respectively), followed by the hepta-thymidine repeat (-51 to -57), upstream from the transcriptional start site. Partially overlapping the heptathymidine repeat site of the sfx1 mutation, we identified an 18-bp element which contained 11 bp identical to the consensus binding site for the E. coli PhoB protein (Fig. 4).

    DISCUSSION
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

The data presented in this report and previous biochemical data (37) suggest that the orfA-pit operon of R. meliloti encodes a phosphate transport system whose expression is regulated by the amount of available inorganic phosphate. The orfA-pit genes are expressed in cells grown in the presence of excess phosphate (2 mM Pi), whereas under Pi limiting conditions orfA-pit transcription is repressed (Fig. 5). Unlike the latter wild-type pattern of orfA-pit regulation, our data show that in phoCDET mutant cells, transcription of the orfA-pit operon is repressed, even in cells cultured with excess Pi (2 mM) (Fig. 5b, data sets 3 and 5). Thus, phoCDET mutants grow poorly in medium containing 2 mM Pi and show a reduced Pi transport phenotype not because they lack the PhoCDET transport system but because orfA-pit expression is repressed. Hence, the Pi assimilation phenotypes resulting from phoCDET mutations are eliminated by secondary mutations such as sfx1 which increase orfA-pit transcription. The fact that sfx1 was identified as a suppressor of the Fix- phenotype of phoCDET mutants reinforces the suggestion that the phoCDET Fix- phenotype is a direct consequence of the Pi assimilation phenotype of these mutants.

In E. coli and Acinetobacter johnsonii, there is physiological evidence that a Pit-like phosphate transport system, which transports Pi as a neutral metal phosphate complex, operates during conditions of Pi excess and that under Pi limiting conditions a separate high-affinity phosphate transport system is employed for Pi uptake (PstSCAB in E. coli) (33-35, 39, 40). While the proposed role for the Pit protein in R. meliloti is similar to that of the E. coli Pit protein, our data clearly suggest that orfA-pit expression is phosphate regulated in R. meliloti whereas the E. coli Pit transport system is believed to be constitutively expressed (40).

Why phoCDET mutations result in repression of orfA-pit expression is unclear. However, we recently showed that this repression is mediated through the transcriptional activator PhoB since mutations which inactivate the phoB gene relieve this repression (1). In wild-type cells, PhoB also appears to mediate the phosphate-dependent regulation of orfA-pit expression, since orfA-pit expression is constitutive in a phoB mutant background (1). In view of the above data, it is particularly interesting to find a PhoB-like binding site in the orfA promoter. In E. coli, genes whose expression is activated by PhoB contain a PhoB binding site in the -35 region of the promoter. Similarly, the R. meliloti phoC promoter contains a PhoB-like binding site in the -35 region, and phoC expression is positively regulated by phoB (2). The PhoB-like binding site of the R. meliloti orfA-pit wild-type promoter is centered at -62.5, and we suggest that this atypical positioning reflects the negative regulation of orfA-pit expression by PhoB.

While the single thymidine deletion (sfx1) mutation increases the basal expression and strength of the orfA-pit promoter, expression is still regulated by the level of available phosphate. Indeed, the sfx1 mutation appears to increase the sensitivity of the orfA-pit promoter to regulation by phosphate, since sfx1-directed orfA-pit transcription was Pi regulated in both the wild-type and phoC backgrounds (Fig. 5b, data sets 2 to 5), whereas (as noted above) wild-type orfA-pit showed only very low expression in the phoC background (Fig. 5b, data sets 3 and 5). Given the location of the sfx1 mutation and PhoB-like binding site (Fig. 4), it is not surprising that sfx1 transcription is still subject to phosphate-dependent regulation. Further studies are clearly required to verify the PhoB binding site and to investigate how the deletion mutation increases the strength of the orfA-pit promoter.

    ACKNOWLEDGMENTS

This work was supported by operating and strategic grants from the Natural Sciences and Engineering Research Council of Canada to T.M.F.

We thank Nathan Falcioni for technical assistance in identifying the sfx3 and sfx4 mutations, Bob Watson for the yeast extract fraction, and all members of the Finan laboratory for comments and discussions.

    FOOTNOTES

* Corresponding author. Mailing address: Department of Biology, McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada L8S 4K1. Phone: (905) 525-9140, ext. 22932. Fax: (905) 522-6066. E-mail: FINAN{at}MCMASTER.CA.

    REFERENCES
Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References

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Journal of Bacteriology, August 1998, p. 4219-4226, Vol. 180, No. 16
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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