Genes Involved in the Carbon Metabolism of Bacteroids. In: Evans HJ, Bottomley PJ, Newton WE, editors. Nitrogen fixation research progress. Dordrecht: Springer Netherlands; 1985. pp. 201-7. [DOI: 10.1007/978-94-009-5175-4_27]

Developmental downregulation of rhizobial genes as a function of symbiosome differentiation in symbiotic root nodules of Pisum sativum

•  The expression of nodA and dctA genes of Rhizobium leguminosarum bv. viciae has been studied in mutant nodules of pea (Pisum sativum L.), blocked at the following developmental stages: infection thread development inside the nodule (Itn); infection droplet differentiation (Idd); bacteroid differentiation after endocytosis (Bad); and nodule persistence (Nop). •  With the use of reporter fusions to these symbiotic bacterial genes it was shown that both nodA and dctA were expressed at all developmental stages, with a pattern similar to that of constitutive, symbiosis-unrelated genes. •  As well as two constitutively expressed genes, both nodA and dctA genes seemed to be subjected to gradual downregulation in nodule bacteria, correlating with the stage of bacteroid differentiation reached. No such effect was observed for the symbiotic, oxygen-regulated fixN gene. The bacteroid development stage also appeared to be related to the ability of bacteria that have been subjected to endocytosis to resume free-living vegetative growth. •  The results support the suggestion that bacteroid differentiation into a nitrogen-fixing, organelle-like form, is a gradual process involving several stages, each controlled by different plant genes.

Control of inducer accumulation plays a key role in succinate-mediated catabolite repression in Sinorhizobium meliloti

The symbiotic, nitrogen-fixing bacterium Sinorhizobium meliloti favors succinate and related dicarboxylic acids as carbon sources. As a preferred carbon source, succinate can exert catabolite repression upon genes needed for the utilization of many secondary carbon sources, including the alpha-galactosides raffinose and stachyose. We isolated lacR mutants in a genetic screen designed to find S. meliloti mutants that had abnormal succinate-mediated catabolite repression of the melA-agp genes, which are required for the utilization of raffinose and other alpha-galactosides. The loss of catabolite repression in lacR mutants was seen in cells grown in minimal medium containing succinate and raffinose and grown in succinate and lactose. For succinate and lactose, the loss of catabolite repression could be attributed to the constitutive expression of beta-galactoside utilization genes in lacR mutants. However, the inactivation of lacR did not cause the constitutive expression of alpha-galactoside utilization genes but caused the aberrant expression of these genes only when succinate was present. To explain the loss of diauxie in succinate and raffinose, we propose a model in which lacR mutants overproduce beta-galactoside transporters, thereby overwhelming the inducer exclusion mechanisms of succinate-mediated catabolite repression. Thus, some raffinose could be transported by the overproduced beta-galactoside transporters and cause the induction of alpha-galactoside utilization genes in the presence of both succinate and raffinose. This model is supported by the restoration of diauxie in a lacF lacR double mutant (lacF encodes a beta-galactoside transport protein) grown in medium containing succinate and raffinose. Biochemical support for the idea that succinate-mediated repression operates by preventing inducer accumulation also comes from uptake assays, which showed that cells grown in raffinose and exposed to succinate have a decreased rate of raffinose transport compared to control cells not exposed to succinate.

Genetic characterization of a Sinorhizobium meliloti chromosomal region in lipopolysaccharide biosynthesis

The genetic characterization of a 5.5-kb chromosomal region of Sinorhizobium meliloti 2011 that contains lpsB, a gene required for the normal development of symbiosis with Medicago spp., is presented. The nucleotide sequence of this DNA fragment revealed the presence of six genes: greA and lpsB, transcribed in the forward direction; and lpsE, lpsD, lpsC, and lrp, transcribed in the reverse direction. Except for lpsB, none of the lps genes were relevant for nodulation and nitrogen fixation. Analysis of the transcriptional organization of lpsB showed that greA and lpsB are part of separate transcriptional units, which is in agreement with the finding of a DNA stretch homologous to a "nonnitrogen" promoter consensus sequence between greA and lpsB. The opposite orientation of lpsB with respect to its first downstream coding sequence, lpsE, indicated that the altered LPS and the defective symbiosis of lpsB mutants are both consequences of a primary nonpolar defect in a single gene. Global sequence comparisons revealed that the greA-lpsB and lrp genes of S. meliloti have a genetic organization similar to that of their homologous loci in R. leguminosarum bv. viciae. In particular, high sequence similarity was found between the translation product of lpsB and a core-related biosynthetic mannosyltransferase of R. leguminosarum bv. viciae encoded by the lpcC gene. The functional relationship between these two genes was demonstrated in genetic complementation experiments in which the S. meliloti lpsB gene restored the wild-type LPS phenotype when introduced into lpcC mutants of R. leguminosarum. These results support the view that S. meliloti lpsB also encodes a mannosyltransferase that participates in the biosynthesis of the LPS core. Evidence is provided for the presence of other lpsB-homologous sequences in several members of the family Rhizobiaceae.

Inactivation and regulation of the aerobic C(4)-dicarboxylate transport (dctA) gene of Escherichia coli

The gene (dctA) encoding the aerobic C(4)-dicarboxylate transporter (DctA) of Escherichia coli was previously mapped to the 79-min region of the linkage map. The nucleotide sequence of this region reveals two candidates for the dctA gene: f428 at 79.3 min and the o157a-o424-o328 (or orfQMP) operon at 79.9 min. The f428 gene encodes a homologue of the Sinorhizobium meliloti and Rhizobium leguminosarum H(+)/C(4)-dicarboxylate symporter, DctA, whereas the orfQMP operon encodes homologues of the aerobic periplasmic-binding protein- dependent C(4)-dicarboxylate transport system (DctQ, DctM, and DctP) of Rhodobacter capsulatus. To determine which, if either, of these loci specify the E. coli DctA system, the chromosomal f428 and orfM genes were inactivated by inserting Sp(r) or Ap(r) cassettes, respectively. The resulting f428 mutant was unable to grow aerobically with fumarate or malate as the sole carbon source and grew poorly with succinate. Furthermore, fumarate uptake was abolished in the f428 mutant and succinate transport was approximately 10-fold lower than that of the wild type. The growth and fumarate transport deficiencies of the f428 mutant were complemented by transformation with an f428-containing plasmid. No growth defect was found for the orfM mutant. In combination, the above findings confirm that f428 corresponds to the dctA gene and indicate that the orfQMP products play no role in C(4)-dicarboxylate transport. Regulation studies with a dctA-lacZ (f428-lacZ) transcriptional fusion showed that dctA is subject to cyclic AMP receptor protein (CRP)-dependent catabolite repression and ArcA-mediated anaerobic repression and is weakly induced by the DcuS-DcuR system in response to C(4)-dicarboxylates and citrate. Interestingly, in a dctA mutant, expression of dctA is constitutive with respect to C(4)-dicarboxylate induction, suggesting that DctA regulates its own synthesis. Northern blot analysis revealed a single, monocistronic dctA transcript and confirmed that dctA is subject to regulation by catabolite repression and CRP. Reverse transcriptase-mediated primer extension indicated a single transcriptional start site centered 81 bp downstream of a strongly predicted CRP-binding site.

Roles of DctA and DctB in signal detection by the dicarboxylic acid transport system of Rhizobium leguminosarum

The dctA gene, coding for the dicarboxylate transport protein, has an inducible promoter dependent on activation by the two-component sensor-regulator pair DctB and DctD. LacZ fusion analysis indicates that there is a single promoter for dctB and dctD. The dctA promoter is also induced by nitrogen limitation, an effect that requires DctB-DctD and NtrC. DctB alone is able to detect dicarboxylates in the absence of DctA and initiate transcription via DctD. However, DctA modifies signal detection by DctB such that in the absence of DctA, the ligand specificity of DctB is broader. dctAp also responds to heterologous induction by osmotic stress in the absence of DctA. This effect requires both DctB and DctD. A transposon insertion in the dctA-dctB intergenic region (dctA101) which locks transcription of dctA at a constitutive level independent of DctB-DctD results in improper signalling by DctB-DctD. Strain RU150, which carries this insertion, is defective in nitrogen fixation (Fix-) and grows very poorly on ammonia as a nitrogen source whenever the DctB-DctD signalling circuit is activated by the presence of a dicarboxylate ligand. Mutation of dctB or dctD in strain RU150 reinstates normal growth on dicarboxylates. This suggests that DctD-P improperly regulates a heterologous nitrogen-sensing operon. Increased expression of DctA, either via a plasmid or by chromosomal duplication, restores control of DctB-DctD and allows strain RU150 to grow on ammonia in the presence of a dicarboxylate. Thus, while DctB is a sensor for dicarboxylates in its own right, it is regulated by DctA. The absence of DctA allows DctB and DctD to become promiscuous with regard to signal detection and cross talk with other operons. This indicates that DctA contributes significantly to the signalling specificity of DctB-DctD and attenuates cross talk with other operons.

Some processes related to nitrogen fixation in nodulated legumes

We have summarized information in four areas of the broad topic of legume- Rhizobium symbiosis. These include: carbon substrates provided to nodule bacteroids by the host, assimilation of fixed nitrogen by the host, O 2 metabolism in legume nodules and involvement of H 2 in nodule metabolism. Although nodules contain a variety of carbon substrates, both biochemical and genetic evidence indicate that C4 dicarboxylates are the major carbon substrates that support N 2 fixation in nodules. The biochemical pathways for utilization of products of N 2 fixation are fairly well understood but relatively little is known about the regulation of the assimilation of fixed nitrogenous compounds at the gene level. Ureides are primary nitrogenous compounds exported from nodules of the tropical legumes. Because the catabolism of these products may involve the hydrolysis of urea by nickel-dependent urease, the possible importance of nickel as a trace element in the nutrition of legumes is raised. The O 2 supply to nodule bacteroids is regulated by a barrier to free-O 2 diffusion and by leghaemoglobin. Progress has been made in understanding of the molecular genetics and biochemistry of leghaemoglobin but little is known about the mechanisms that control the physical barrier to O 2 diffusion. Legume nodules contain mechanisms for the disposition of peroxide and free radicals of oxygen. The importance of these systems as protective mechanisms for the O 2 -labile nitrogenase is discussed. Some strains of Rhizobium form nodules which recycle the H 2 produced as a byproduct of N 2 fixation. The genes necessary for H 2 oxidation have been cloned and transferred within and among species of Rhizobium . The advantages and disadvantages of H 2 recycling in legume nodules are discussed.

Rhizobium meliloti DctD, a sigma 54-dependent transcriptional activator, may be negatively controlled by a subdomain in the C-terminal end of its two-component receiver module

Rhizobium meliloti DctD is believed to have three functional domains: an N-terminal, two-component receiver domain; and like other sigma 54-dependent activators, C-terminal and central domains for DNA binding and transcription activation. We have characterized a progressive series of N-terminal deletions of R. meliloti DctD. The N-terminal domain was not needed for binding the dctA upstream activation sequence. Only 25% of the C-terminal end of the receive domain was needed to significantly inhibit the central domain, and proteins lacking up to 60% of the N-terminal end of the receiver domain were 'inducible' in R. meliloti cells. We hypothesize that the N-terminal two-thirds of the DctD receiver domain augments and controls an adjacent subdomain for inhibiting the central domain.

Symbiotic nitrogen fixation by a nifA deletion mutant of Rhizobium meliloti: the role of an unusual ntrC allele

In the N2-fixing alfalfa symbiont Rhizobium meliloti, the three sigma 54 (NTRA)-dependent positively acting regulatory proteins NIFA, NTRC, and DCTD are required for activation of promoters involved in N2 fixation (pnifHDKE and pfixABCX), nitrogen assimilation (pglnII), and C4-dicarboxylate transport (pdctA), respectively. Here, we describe an allele of ntrC which results in the constitutive activation of the above NTRC-, NIFA-, and DCTD-regulated promoters. The expression and activation of wild-type NTRC occur in response to nitrogen availability, whereas in cells carrying the ntrC283 allele, the NTRC283 protein appears constitutively active and is constitutively expressed. The ntrC283 allele was shown to carry a single mutation resulting in the replacement of an Asp by a Tyr residue in the helix-turn-helix motif of ntrC283. Introduction of the ntrC283 allele into a nifA deletion mutant restores the N2-fixation ability to 70 to 80% of the wild-type level. Thus, the nifA gene is dispensable for symbiotic N2 fixation.

Negative regulation of sigma 54-dependent dctA expression by the transcriptional activator DctD

In Rhizobium meliloti, the presence of the C4-dicarboxylate transport protein DctA is required for symbiotic N2 fixation in alfalfa root nodules. Expression of dctA is inducible and is mediated by a sensor and activator gene pair encoded by dctB and dctD. In the presence of C4-dicarboxylates, the DCTB sensor protein is believed to phosphorylate and activate DCTD, which in turn activates transcription at the sigma 54-dependent dctA promoter. Here, we present evidence that in addition to activating dctA transcription, DCTD can also repress expression of dctA. By employing an ntrC allele, ntrC283, whose product appears to activate dctA transcription independently of DCTD, we found that while ntrC283 leads to constitutive dctA expression in the absence of dctB and dctD, in a dctB+ dctD+ ntrC283 background high-level expression of dctA occurred in succinate but not in glucose-grown cells. This result suggested that in uninduced cells, inactive DCTD binds to the dctA promoter and prevents its activation by NTRC283. Consistent with the latter interpretation was the observation that overexpression of DCTD from a plasmid promoter prevents dctA expression and results in a Dct- phenotype. Moreover the Dct- phenotype resulting from the overexpression of dctD was dominant to ntrC283. Results from studies of the ability of ntrC283 to suppress the Dct- phenotype of dctB alleles, together with the finding that the Fix- phenotype of a particular dctB allele was dctD dependent, suggest that in particular dctB alleles, sufficient dctD transcription occurs such that the resulting inactive DCTD prevents activation of dctA transcription by NtrC283 or alternate symbiotic regulators. The latter suggestion is supported by the observation that in symbiosis, R. meliloti strains in which DCTD was overexpressed formed nodules which failed to fix nitrogen.

Tandem DctD-binding sites of the Rhizobium meliloti dctA upstream activating sequence are essential for optimal function despite a 50- to 100-fold difference in affinity for DctD

The Rhizobium meliloti genes dctB and dctD positively regulate the expression of dctA, which encodes a C4-dicarboxylate transport protein. Here we characterize an element (UAS) located upstream of dctA that has tandem binding sites for the dctD gene product (DctD). At relatively low concentrations of active DctD, the element activated dctA transcription, but at relatively high concentrations of DctD it was inhibitory. The UAS failed to function when placed further upstream of dctA. Both DctD-binding sites were required for optimal UAS function, despite a 50- to 100-fold difference in binding affinities. Moving the promoter distal binding site 5 bp further upstream was functionally equivalent to its deletion. Based on these data, we hypothesize that the sigma 54-dependent activator DctD binds co-operatively to the R. meliloti dctA UAS, and that occupancy of both sites is required for maximal activation of dctA.

The central domain of Rhizobium leguminosarum DctD functions independently to activate transcription

Sigma 54-dependent transcriptional activators such as Escherichia coli NtrC, Rhizobium meliloti NifA, and Rhizobium leguminosarum DctD share similar central and carboxy-terminal domains but differ in the structure and function of their amino-terminal domains. We have deleted the amino-terminal and carboxy-terminal domains of R. leguminosarum DctD and have demonstrated that the central domain of DctD, like that of NifA, is transcriptionally competent.

Rhizobium meliloti and Rhizobium leguminosarum dctD gene products bind to tandem sites in an activation sequence located upstream of sigma 54-dependent dctA promoters

Free-living rhizobia transport external C4-dicarboxylates to use as sole carbon sources, and uptake of these compounds is essential for nitrogen fixation by rhizobial bacteroids. In both Rhizobium leguminosarum and Rhizobium meliloti, the genes dctB and dctD are believed to form an ntrB/ntrC-like two-component system which regulates the synthesis of a C4-dicarboxylate transport protein encoded by dctA. Here we confirm the identity of sigma 54-dependent promoters previously hypothesized for the R. leguminosarum and R. meliloti dctA genes and demonstrate that repeated, partial dyad symmetry elements located about 75 base pairs upstream of each promoter are essential for fully regulated transcription. Furthermore, we show that both repeats bound dctD protein and that together they resulted in succinate-sensitive transcription when placed upstream of another sigma 54 consensus promoter, that of R. meliloti nifH.

Nucleotide sequence of the regulatory nifA gene of Rhizobium leguminosarum PRE: transcriptional control sites and expression in Escherichia coli

We report the sequence of the regulatory nifA gene of Rhizobium leguminosarum PRE. The transcription initiation and termination sites of nifA were mapped and a potential promoter and a rho-independent terminator identified. The nifA gene has two possible translation start sites, both of which are used in an Escherichia coli background, resulting in proteins with apparent molecular weights of 58 kD and 57 kD; initiation at the second site is preferred over initiation at the first. The nifA-nifB intergenic region contains an rpoN-dependent promoter for the nifB gene but no consensus upstream activator sequence (UAS). A potential DNA-binding domain, consisting of two alpha-helices separated by a four-amino-acid linker, is located at the C-terminal end of the NifA amino acid sequence.

Identification and sequence analysis of the Rhizobium meliloti dctA gene encoding the C4-dicarboxylate carrier

Transposon Tn5-induced C4-dicarboxylate transport mutants of Rhizobium meliloti 2011 which could be complemented by cosmid pRmSC121 were subdivided into two classes. Class I mutants (RMS37 and RMS938) were defective in symbiotic C4-dicarboxylate transport and in nitrogen fixation. They were mutated in the structural gene dctA, which codes for the C4-dicarboxylate carrier. Class II mutants (RMS11, RMS16, RMS17, RMS24, and RMS31) expressed reduced activity in symbiotic C4-dicarboxylate transport and in nitrogen fixation. These mutants were mutated in regulatory dct genes which do not play an essential role in the symbiotic state. Thin sections of alfalfa nodules induced by the wild type and class I and class II mutants were analyzed by light microscopy. Class mutants induced typical Fix- nodules, showing a large senescent zone, whereas nodules induced by class II mutants only differed in an enhanced content of starch granules compared with wild-type nodules. Class I mutants could be complemented by a 2.1-kilobase SalI-HindIII subfragment of cosmid pRmSC121. DNA sequencing of this fragment resulted in the identification of an open reading frame, which was designated dctA because Tn5 insertion sites of the class I mutants mapped within this coding region. The dctA gene was preceded by a nif consensus promoter and an upstream NifA-binding element. Upstream of the dctA promoter, the 5' end of the R. meliloti dctB gene could be localized. The amino acid sequence of the N-terminal part of the R. meliloti DctB protein shared 49% homology with the corresponding part of the R. leguminosarum DctB protein. The DctA protein consisted of 441 or 453 amino acids due to two possible ATG start codons, with calculated molecular masses of 46.1 and 47.6 kilodaltons, respectively. The hydrophobicity plot suggests that DctA is a membrane protein with several membrane passages. The amino acid sequences of the R. meliloti and the R. leguminosarum DctA proteins were highly conserved (82%).

Conservation between coding and regulatory elements of Rhizobium meliloti and Rhizobium leguminosarum dct genes

Complementation of Rhizobium leguminosarum dct mutants with a cosmid bank yielded Rhizobium meliloti homologs of the dctA, dctB, and dctD genes. The genes dctB and dctD are thought to form a two-component system which responds to the presence of C4-dicarboxylates to regulate expression of a transport protein encoded by dctA. DNA sequence analysis showed that dct coding and intergenic regions, including putative binding sites for the dctD protein and sigma 54-RNA polymerase, were highly conserved between these two Rhizobium species. Mutation of R. meliloti dctD showed that it was not essential for symbiotic nitrogen fixation but was needed for growth on succinate and the expression of a dctA-lacZ fusion gene in free-living cells. Hybridization of R. meliloti genomic DNA with probes representing the central portion of dctD potentially identified more than 20 similar regulatory genes, all of which are likely to depend upon the alternative sigma factor encoded by rpoN and stimulate transcription in a manner very similar to ntrC activation of glnA in enteric bacteria.

Dual control of the Bradyrhizobium japonicum symbiotic nitrogen fixation regulatory operon fixR nifA: analysis of cis- and trans-acting elements

Aerobic expression of the fixR nifA operon in Bradyrhizobium japonicum was shown to depend on a cis-acting, promoter-upstream DNA sequence located between the -24/-12 promoter and position -86 relative to the transcription start site. An adenine at position -66 was essential for maximal expression. A chromosomal deletion of the upstream activator sequence (UAS) led to a symbiotically defective phenotype which was typical of nifA mutants. B. japonicum crude extracts contained a protein that bound to the UAS. By using chromosomally integrated fixR-lacZ fusions, the level of expression of the fixR nifA operon was found to be fivefold higher under reduced oxygen tension than under aerobiosis. This increase was due to autoactivation by the NifA protein and was partly independent of the UAS. Based on these data, we propose a model for the regulation of nitrogen fixation genes in B. japonicum that involves dual positive control of the fixR nifA operon. At high oxygen concentrations, the operon is expressed at a moderate level, subject to activation by the binding of a trans-acting factor to the UAS. Under such conditions, the nifA gene product is known to be inactive. At very low oxygen concentrations--a condition favorable to NifA activity--the NifA protein is the trans-acting factor which (i) enhances the level of fixR nifA expression (and hence its own synthesis) and (ii) activates other nif and fix genes.

Analysis of C4-dicarboxylate transport genes in Rhizobium meliloti

A 5.1 kbp DNA fragment was isolated which complemented C4-dicarboxylate transport mutants (dct) of Rhizobium meliloti. Characterization of this fragment by subcloning, transposon mutagenesis, and complementation analysis revealed three loci, designated dctA, dctB, and dctD. TnphoA-generated alkaline phosphatase fusions to dctA suggested that this gene encodes the structural transport protein and allowed the determination of its direction of transcription. Analysis of the fusions in various mutant backgrounds demonstrated that dctB, dctD, and ntrA products are required for dctA expression. The dctA fusion was constitutively expressed in a dctA mutant background, but was not expressed in dctA dctB or dctA dctD double mutants. This suggests that the constitutive expression in a dctA mutant background is mediated through dctB and dctD. Three independent second-site Dct+ revertant mutations in ntrA mutant strains mapped to the dct locus. Succinate transport in these revertant strains was constitutive, whereas in the wild type, succinate transport was inducible. These results are consistent with the direct requirement of the ntrA gene product for dctA expression. Alfalfa plants inoculated with the dctB and dctD mutants showed reduced nitrogen-fixing activity. Nodules induced by dctA mutants failed to fix nitrogen. These symbiotic phenotypes are consistent with previous suggestions that dctA expression in bacteroids can occur independently of dctB and dctD.

Induction of Symbiotically Defective Auxotrophic Mutants of Rhizobium fredii HH303 by Transposon Mutagenesis

Symbiotically defective auxotrophic mutants were isolated by transposon Tn 5 mutagenesis of Rhizobium fredii HH303, a fast-growing microsymbiont of North American commercial soybean cultivars such as Glycine max cv. Williams. Three different Tn 5 -carrying suicide vectors, pBLK1-2, pSUP1011, and pGS9, were used for mutagenesis with transposition frequencies of 4 × 10 −5 , 3 × 10 −6 , and 1 × 10 −6 , respectively, while the frequency of background mutation resistant to 500 μg of kanamycin per ml was 1 × 10 −8 . From 2,600 Tn 5 -induced mutants, 14 auxotrophic mutants were isolated and classified in seven groups including adenosine (four), aspartate (two), cysteine or methionine (two), isoleucine and valine (two), nicotinic acid (one), pantothenic acid (one), and uracil (two). All the auxotrophs induced nodulation on soybean, but the symbiotic effectiveness of each mutant was different. Three auxotrophs (two cysteine or methionine and one pantothenic acid) formed effective nodules similar to those of the wild type. Three auxotrophs (one nicotinic acid and two aspartate) produced mature nodules like those of the wild type, but the nodules lacked the characteristic pink color inside and were unable to fix nitrogen. Four auxotrophs (two adenosine and two uracil) induced pseudonodules unable to fix nitrogen. The other four auxotrophs repeatedly induced both effective and ineffective nodules, but bacteroids isolated from the effective nodules were prototrophic revertants. The symbiotic phenotype and the degree of effectiveness of the auxotrophic mutants varied with the type of mutation.

Dicarboxylic acid transport in Bradyrhizobium japonicum: use of Rhizobium meliloti dct gene(s) to enhance nitrogen fixation

A recombinant plasmid encoding Rhizobium meliloti sequences involved in dicarboxylic acid transport (plasmid pRK290:4:46) (E. Bolton, B. Higgisson, A. Harrington, and F. O'Gara, Arch. Microbiol. 144:142-146, 1986) was used to study the relationship between dicarboxylic acid transport and nitrogen fixation in Bradyrhizobium japonicum. The expression of the dct sequences on plasmid pRK290:4:46 in B. japonicum CJ1 resulted in increased growth rates in media containing dicarboxylic acids as the sole source of carbon. In addition, strain CJ1(pRK290:4:46) exhibited enhanced succinate uptake activity when grown on dicarboxylic acids under aerobic conditions. Under free-living nitrogen-fixing conditions, strain CJ1(pRK290:4:46) exhibited higher nitrogenase (acetylene reduction) activity compared with that of the wild-type strain. This increase in nitrogenase activity also correlated with an enhanced dicarboxylic acid uptake rate under these microaerobic conditions. The regulation of dicarboxylic acid transport by factors such as metabolic inhibitors and the presence of additional carbon sources was similar in both the wild-type and the engineered strains. The implications of increasing nitrogenase activity through alterations in the dicarboxylic acid transport system are discussed.

Rhizobium meliloti ntrA (rpoN) gene is required for diverse metabolic functions

We report the identification and cloning of an ntrA-like (glnF rpoN) gene of Rhizobium meliloti and show that the R. meliloti ntrA product (NtrA) is required for C4-dicarboxylate transport as well as for nitrate assimilation and symbiotic nitrogen fixation. DNA sequence analysis showed that R. meliloti NtrA is 38% homologous with Klebsiella pneumoniae NtrA. Subcloning and complementation analysis suggested that the R. meliloti ntrA promoter lies within 125 base pairs of the initiation codon and may be constitutively expressed.