[Show abstract][Hide abstract] ABSTRACT: Rhodobacter capsulatus fixes atmospheric dinitrogen via two nitrogenases, Mo- and Fe-nitrogenase, which operate under different conditions. Here,
we describe the functions in nitrogen fixation and regulation of the rcc00574 (cooA) and rcc00575 (cowN) genes, which are located upstream of the structural genes of Mo-nitrogenase, nifHDK. Disruption of cooA or cowN specifically impaired Mo-nitrogenase-dependent growth at carbon monoxide (CO) concentrations still tolerated by the wild
type. The cooA gene was shown to belong to the Mo-nitrogenase regulon, which is exclusively expressed when ammonium is limiting. Its expression
was activated by NifA1 and NifA2, the transcriptional activators of nifHDK. AnfA, the transcriptional activator of Fe-nitrogenase genes, repressed cooA, thereby counteracting NifA activation. CooA activated cowN expression in response to increasing CO concentrations. Base substitutions in the presumed CooA binding site located upstream
of the cowN transcription start site abolished cowN expression, indicating that cowN regulation by CooA is direct. In conclusion, a transcription factor-based network controls cowN expression to protect Mo-nitrogenase (but not Fe-nitrogenase) under appropriate conditions.
Journal of Bacteriology 07/2014; 196(19). DOI:10.1128/JB.01754-14 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rhodobacter capsulatus is able to grow with N2 as the sole nitrogen source using either a molybdenum-dependent or a molybdenum-free iron-only nitrogenase, whose expression is strictly inhibited by ammonium. Disruption of the fdxD gene, which is located directly upstream of the Mo-nitrogenase genes, nifHDK, abolished diazotrophic growth via Mo-nitrogenase at oxygen concentrations still tolerated by the wild type, thus demonstrating the importance of FdxD under semi-aerobic conditions. By contrast, FdxD was not beneficial for diazotrophic growth depending on Fe-nitrogenase. These findings suggest that the 2Fe2S ferredoxin FdxD specifically supports the Mo-nitrogenase system probably by protecting Mo-nitrogenase against oxygen as previously shown for its Azotobacter vinelandii counterpart, FeSII. Expression of fdxD occurred under nitrogen fixing conditions but not in the presence of ammonium. Expression of fdxD strictly required NifA1 and NifA2, the transcriptional activators of the Mo-nitrogenase genes, but not AnfA, the transcriptional activator of the Fe-nitrogenase genes. Expression of the fdxD and nifH genes as well as the FdxD and NifH protein levels increased with increasing molybdate concentrations. Molybdate induction of fdxD was independent of the molybdate-sensing regulators MopA and MopB, which repress anfA transcription at micromolar molybdate concentrations. In this report we demonstrate the physiological relevance of an fesII-like gene, fdxD, and show that the cellular nitrogen and molybdenum status are integrated to control its expression.
Journal of bacteriology 11/2013; 196(3). DOI:10.1128/JB.01235-13 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Copper is an essential cofactor of various enzymes, but free copper is highly toxic to living cells. To maintain cellular metabolism at different ambient copper concentrations, bacteria have evolved specific copper homeostasis systems that mostly act as defence mechanisms. As well as under free-living conditions, copper defence is critical for virulence in pathogenic bacteria. Most bacteria synthesize P-type copper export ATPases as principal defence determinants when copper concentrations exceed favourable levels. In addition, many bacteria utilize resistance-nodulation-cell division (RND)-type efflux systems and multicopper oxidases to cope with excess copper. This review summarizes our current knowledge on copper-sensing transcriptional regulators, which we assign to nine different classes. Widespread one-component regulators are CueR, CopY and CsoR, which were initially identified in Escherichia coli, Enterococcus hirae and Mycobacterium tuberculosis, respectively. CueR activates homeostasis gene expression at elevated copper concentrations, while CopY and CsoR repress their target genes under copper-limiting conditions. Besides these one-component systems, which sense the cytoplasmic copper status, many Gram-negative bacteria utilize two-component systems, which sense periplasmic copper concentrations. In addition to these well-studied transcriptional factors, copper control mechanisms acting at the post-transcriptional and the post-translational levels will be discussed.
[Show abstract][Hide abstract] ABSTRACT: To identify copper homeostasis genes in Rhodobacter capsulatus, we performed random transposon Tn5 mutagenesis. Screening of more than 10,000 Tn5 mutants identified tellurite resistance gene trgB as a so far unrecognized major copper tolerance determinant. The trgB gene is flanked by tellurite resistance gene trgA and cysteine synthase gene cysK2. While growth of trgA mutants was only moderately restricted by tellurite, trgB and cysK2 mutants were severely affected by tellurite, which implies that viability under tellurite stress requires increased cysteine levels. Mutational analyses revealed that trgB was the only gene in this chromosomal region conferring cross-tolerance towards copper. Expression of the monocistronic trgB gene required promoter elements overlapping the trgA coding region as shown by nested deletions. Neither copper nor tellurite affected trgB transcription as demonstrated by reverse transcriptase PCR and trgB-lacZ fusions. Addition of tellurite or copper gave rise to increased cellular tellurium and copper concentrations, respectively, as determined by inductively coupled plasma-optical emission spectroscopy. By contrast, cellular iron concentrations remained fairly constant irrespective of tellurite or copper addition. This is the first study demonstrating a direct link between copper and tellurite response in bacteria.
Biology of Metals 07/2012; 25(5):995-1008. DOI:10.1007/s10534-012-9566-2 · 2.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The copper-regulated Rhodobacter capsulatus cutO (multicopper oxidase) gene confers copper tolerance and is carried in the tricistronic orf635-cutO-cutR operon. Transcription of cutO strictly depends on the promoter upstream of orf635, as demonstrated by lacZ reporter fusions to nested promoter fragments. Remarkably, orf635 expression was not affected by copper availability, whereas cutO and cutR were expressed only in the presence of copper. Differential regulation was abolished by site-directed mutations within the orf635-cutO intergenic region, suggesting that this region encodes a copper-responsive mRNA element. Bioinformatic predictions and RNA structure probing experiments revealed an intergenic stem-loop structure as the candidate mRNA element. This is the first posttranscriptional copper response mechanism reported in bacteria.
Journal of bacteriology 01/2012; 194(8):1849-59. DOI:10.1128/JB.06274-11 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The LysR-type regulator MopB represses transcription of several target genes (including the nitrogen-fixation gene anfA) in Rhodobacter capsulatus at high molybdenum concentrations. In this study, the isolated DNA-binding domain of MopB (MopBHTH) was overexpressed in Escherichia coli. Purified MopBHTH bound the anfA promoter as shown by DNA mobility-shift assays, demonstrating the function of the isolated regulator domain. MopBHTH was crystallized using the sitting-drop vapour-diffusion method in the presence of 0.2 M lithium sulfate, 0.1 M phosphate/citrate pH 4.2, 20%(w/v) PEG 1000 at 291 K. The crystal belonged to space group P3(1)21 or P3(2)21, with unit-cell parameters a=b=61.84, c=139.64 Å, α=β=90, γ=120°, and diffracted to 3.3 Å resolution at a synchrotron source.
Acta Crystallographica Section F Structural Biology and Crystallization Communications 03/2011; 67(Pt 3):377-9. DOI:10.1107/S1744309110054710 · 0.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Molybdenum (Mo) is an important trace element that is toxic at high concentrations. To resolve the mechanisms underlying Mo toxicity, Rhodobacter capsulatus mutants tolerant to high Mo concentrations were isolated by random transposon Tn5 mutagenesis. The insertion sites of six independent isolates mapped within the same gene predicted to code for a permease of unknown function located in the cytoplasmic membrane. During growth under Mo-replete conditions, the wild-type strain accumulated considerably more Mo than the permease mutant. For mutants defective for the permease, the high-affinity molybdate importer ModABC, or both transporters, in vivo Mo-dependent nitrogenase (Mo-nitrogenase) activities at different Mo concentrations suggested that ModABC and the permease import molybdate in nanomolar and micromolar ranges, respectively. Like the permease mutants, a mutant defective for ATP sulfurylase tolerated high Mo concentrations, suggesting that ATP sulfurylase is the main target of Mo inhibition in R. capsulatus. Sulfate-dependent growth of a double mutant defective for the permease and the high-affinity sulfate importer CysTWA was reduced compared to those of the single mutants, implying that the permease plays an important role in sulfate uptake. In addition, permease mutants tolerated higher tungstate and vanadate concentrations than the wild type, suggesting that the permease acts as a general oxyanion importer. We propose to call this permease PerO (for oxyanion permease). It is the first reported bacterial molybdate transporter outside the ABC transporter family.
Journal of bacteriology 11/2010; 192(22):5943-52. DOI:10.1128/JB.00742-10 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Either of two related molybdenum-responsive regulators, MopA and MopB, of Rhodobacter capsulatus is sufficient to repress the nitrogen-fixation gene anfA. In contrast, MopA (but not MopB) activates mop, which codes for a molybdate (Mo)-binding molbindin. Both regulators bind to conserved cis-regulatory elements called Mo-boxes. Single-base substitution of two highly conserved nucleotides within the anfA-Mo-box (T21C and C24T) had little effect on regulator binding and anfA expression as shown by DNA mobility shift assays and reporter gene fusions, respectively. In contrast to C24T, mutation C24A strongly diminished binding and repression by MopA and MopB, showing that different nucleotide substitutions at the same position may have very different effects. A triple mutation destroying the left half-site of the mop-Mo-box completely abolished mop expression by MopA, demonstrating the importance of the mop-Mo-box for mop activation. Two point mutations (T23A and T24C) still allowed binding by MopA, but abolished mop activation, most likely because these nucleotides overlap with the RNA polymerase-binding site. A mutant mop promoter, in which the mop-Mo-box was exchanged against the anfA-Mo-box, allowed activation by MopA, showing that a former repressor-binding site may act as an activator-binding site depending on its location relative to the other promoter elements.
[Show abstract][Hide abstract] ABSTRACT: The vast majority of the purple nonsulfur photosynthetic bacteria are diazotrophs, but the details of the complex regulation of the nitrogen fixation process are well understood only for a few species. Here we review what is known of the well-studied Rhodobacter capsulatus, which contains two different nitrogenases, a standard Mo-nitrogenase and an alternative Fe-nitrogenase, and which has overlapping transcriptional control mechanisms with regard to the presence of fixed nitrogen, oxygen, and molybdenum as well as the capability for the post-translational control of both nitrogenases in response to ammonium. R. capsulatus has two PII proteins, GlnB and GlnK, which play key roles in nitrogenase regulation at each of three different levels: activation of transcription of the nif-specific activator NifA, the post-translational control of NifA activity, and the regulation of nitrogenase activity through either ADP-ribosylation of NifH or an ADP-ribosylation-independent pathway. We also review recent work that has led to a detailed characterization of the molybdenum transport and regulatory system in R. capsulatus that ensures activity of the Mo-nitrogenase and repression of the Fe-nitrogenase, down to extremely low levels of molybdenum.
Advances in Experimental Medicine and Biology 01/2010; 675:49-70. DOI:10.1007/978-1-4419-1528-3_4 · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The phototrophic purple bacterium Rhodobacter capsulatus encodes two transcriptional regulators, MopA and MopB, with partially overlapping and specific functions in molybdate-dependent
gene regulation. Both MopA and MopB consist of an N-terminal DNA-binding helix-turn-helix domain and a C-terminal molybdate-binding
di-MOP domain. They formed homodimers as apo-proteins and in the molybdate-bound state as shown by yeast two-hybrid (Y2H)
studies, glutaraldehyde cross-linking, gel filtration chromatography, and copurification experiments. Y2H studies suggested
that both the DNA-binding and the molybdate-binding domains contribute to dimer formation. Analysis of molybdate binding to
MopA and MopB revealed a binding stoichiometry of four molybdate oxyanions per homodimer. Specific interaction partners of
MopA and MopB were the molybdate transporter ATPase ModC and the molbindin-like Mop protein, respectively. Like other molbindins,
the R. capsulatus Mop protein formed hexamers, which were stabilized by binding of six molybdate oxyanions per hexamer. Heteromer formation
of MopA and MopB was shown by Y2H studies and copurification experiments. Reporter gene activity of a strictly MopA-dependent
mop-lacZ fusion in mutant strains defective for either mopA, mopB, or both suggested that MopB negatively modulates expression of the mop promoter. We propose that depletion of the active MopA homodimer pool by formation of MopA-MopB heteromers might represent
a fine-tuning mechanism controlling mop gene expression.
Journal of bacteriology 07/2009; 191(16):5205-15. DOI:10.1128/JB.00526-09 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rhodobacter capsulatus can efficiently grow with taurine as the sole sulfur source. The products of the tpa-tauR-xsc gene region are essential for this activity. TauR, a MocR-like member of the GntR superfamily of transcriptional regulators, activates tpa transcription, as shown by analysis of wild-type and tauR mutant strains carrying a tpa-lacZ reporter fusion. Activation of the tpa promoter requires taurine but is not inhibited by sulfate, which is the preferred sulfur source. TauR directly binds to the tpa promoter, as demonstrated by DNA mobility shift assays. As expected for a transcriptional activator, the TauR binding site is located upstream of the transcription start site, which has been determined by primer extension. Site-directed promoter mutations reveal that TauR binds to direct repeats, an unusual property that has to date been shown for only one other member of the MocR subfamily, namely, GabR from Bacillus subtilis. In contrast, all other members of the GntR family analyzed so far bind to inverted repeats.
Journal of bacteriology 02/2008; 190(2):487-93. DOI:10.1128/JB.01510-07 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the photosynthetic bacterium Rhodobacter capsulatus, the post-translational regulation of the MoFe nitrogenase is deficient in a strain with a kanamycin insertion in amtB, a gene coding for an ammonium channel (Yakunin and Hallenbeck, 2002). GlnB and GlnK, two homologues of the PII protein, are also involved in this regulatory process since there is no nitrogenase switch-off in a glnB-/glnK- double mutant strain (Drepper et al., 2003). Here, we have examined the effect of single mutations in either GlnB or GlnK
on nitrogenase regulation. There was no nitrogenase switch-off in the glnB strain when subjected to the addition of 200 μM NH4+. When ADP-ribosylation of the Fe protein in response to a 50 mM NH4+ addition was examined, only a moderate response after 80 min was noted for the glnB strain, whereas the wild-type strain showed appreciable Fe-protein modification after 40 min, and even more substantial modification
after 80 min. Thus, GlnB appears necessary for nitrogenase switch-off and Fe-protein modification in R. capsulatus.
Biological Nitrogen Fixation: Towards Poverty Alleviation through Sustainable Agriculture, 12/2007: pages 353-354;
[Show abstract][Hide abstract] ABSTRACT: Nitrogen fixation is the process of reducing atmospheric dinitrogen to ammonia, carried out by the enzyme nitrogenase using
a source of reductant and energy (ATP). Early studies in the field of nitrogen fixation involved the discovery of nitrogenase
and understanding the physiological properties of diazotrophs, including the evidence that most purple photosynthetic bacteria
have this capability. More recently, at the forefront has been the understanding of diverse regulatory signals that turn on
and off nitrogen fixation genes as well as the structure and mechanisms of nitrogenase. Here is presented the unique and common
aspects of regulation of nitrogen fixation in the purple photosynthetic bacteria. The most studied model is Rhodobacter capsulatus, where three levels of control are apparent. The first senses nitrogen and ATP, mediated by the Ntr system. After transcriptional
activation by NtrC, the second involves oxygen and more nitrogen sensing via the NifA-type transcriptional activator. The
final level of control resides at the activity of nitrogenase, which is modulated by the DraT and DraG system in response
to nitrogen and other signals. Some interplay exists between nitrogen fixation circuitry and general regulatory systems for
light and redox sensors of purple phototrophs. In summary, the regulatory circuitry is complex; sensors respond to ATP, nitrogen
status, oxygen level, light, and the availability of molybdenum and iron, so that nitrogenase is only expressed and active
under conditions that make physiological sense. Also discussed is the analysis of other purple phototrophs, the presence of
alternative nitrogenases, as well as what the future may hold in this area. For example, light energy is ultimately used to
fix nitrogen in these prokaryotes. Given the increased costs and environmental concerns for fertilizers, what is the potential
for harnessing these capabilities and what are the possible hurdles, not the least of which is the need to control feedback
regulations from all these signals?
[Show abstract][Hide abstract] ABSTRACT: Both Rhodobacter capsulatus PII homologs GlnB and GlnK were found to be necessary for the proper regulation of nitrogenase activity and modification
in response to an ammonium shock. As previously reported for several other bacteria, ammonium addition triggered the AmtB-dependent
association of GlnK with the R. capsulatus membrane. Native polyacrylamide gel electrophoresis analysis indicates that the modification/demodification of one PII homolog
is aberrant in the absence of the other. In a glnK mutant, more GlnB was found to be membrane associated under these conditions. In a glnB mutant, GlnK fails to be significantly sequestered by AmtB, even though it appears to be fully deuridylylated. Additionally,
the ammonium-induced enhanced sequestration by AmtB of the unmodifiable GlnK variant GlnK-Y51F follows the wild-type GlnK
pattern with a high level in the cytoplasm without the addition of ammonium and an increased level in the membrane fraction
after ammonium treatment. These results suggest that factors other than PII modification are driving its association with
AmtB in the membrane in R. capsulatus.
Journal of Bacteriology 09/2007; 189(16):5850-9. DOI:10.1128/JB.00680-07 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The phototrophic purple bacterium Rhodobacter capsulatus encodes two similar but functionally not identical molybdenum-dependent regulator proteins (MopA and MopB), which are known to replace each other in repression of the modABC genes (coding for an ABC-type high-affinity Mo transport system) and anfA (coding for the transcriptional activator of Fe-nitrogenase genes). We identified further Mo-regulated (mor) genes coding for a putative ABC-type transport system of unknown function (MorABC) and a putative Mo-binding protein (Mop). The genes coding for MopA and the ModABC transporter form part of a single transcriptional unit, mopA-modABCD, as shown by reverse transcriptase PCR. Immediately upstream of mopA and transcribed in the opposite direction is mopB. The genes coding for the putative MorABC transporter belong to two divergently transcribed operons, morAB and morC. Expression studies based on lacZ reporter gene fusions in mutant strains defective for either MopA, MopB, or both revealed that the regulators substitute for each other in Mo-dependent repression of morAB and morC. Specific Mo-dependent activation of the mop gene by MopA, but not MopB, was found to control the putative Mo-binding protein. Both MopA and MopB are thought to bind to conserved DNA sequences with dyad symmetry in the promoter regions of all target genes. The positions of these so-called Mo boxes relative to the transcription start sites (as determined by primer extension analyses) differed between Mo-repressed genes and the Mo-activated mop gene. DNA mobility shift assays showed that MopA and MopB require molybdenum to bind to their target sites with high affinity.
Journal of Bacteriology 01/2007; 188(24):8441-51. DOI:10.1128/JB.01188-06 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rhodobacter capsulatus NtrB/NtrC two-component regulatory system controls expression of genes involved in nitrogen metabolism including urease and nitrogen fixation genes. The ntrY-ntrX genes, which are located immediately downstream of the nifR3-ntrB-ntrC operon, code for a two-component system of unknown function. Transcription of ntrY starts within the ntrC-ntrY intergenic region as shown by primer extension analysis, but maximal transcription requires, in addition, the promoter of the nifR3-ntrB-ntrC operon. While ntrB and ntrY single mutant strains were able to grow with either urea or N2 as sole nitrogen source, a ntrB/ntrY double mutant (like a ntrC-deficient strain) was no longer able to use urea or N2. These findings suggest that the histidine kinases NtrB and NtrY can substitute for each other as phosphodonors towards the response regulator NtrC.
[Show abstract][Hide abstract] ABSTRACT: The cutO gene of the photosynthetic purple bacterium Rhodobacter capsulatus codes for a multicopper oxidase as demonstrated by the ability of the recombinant Strep-tagged protein to oxidize several mono- and diphenolic compounds known as substrates of Escherichia coli CueO and multicopper oxidases from other organisms. The R. capsulatus cutO gene was shown to form part of a tri-cistronic operon, orf635-cutO-cutR. Expression of the cutO operon was repressed under low copper conditions by the product of the cutR gene. CutO conferred copper tolerance not only under aerobic conditions, as described for the well-characterized E. coli multicopper oxidase CueO, but also under anaerobic conditions.
[Show abstract][Hide abstract] ABSTRACT: Growth of Rhodobacter capsulatus with molecular dinitrogen as the sole N source via the alternative Fe-only nitrogenase requires all seven gene products of the anfHDGK-1-2-3 operon. In contrast to mutant strains carrying lesions in the structural genes of nitrogenase (anfH, anfD, anfG, and anfK), strains defective for either anf1, anf2, or anf3 are still able to reduce the artificial substrate acetylene, although with diminished activity. To obtain further information on the role of Anf1, we screened an R. capsulatus genomic library designed for use in yeast two-hybrid studies with Anf1 as bait. Two genes, which we propose to call ranR and ranT (for genes related to alternative nitrogenase), coding for products that interact with Anf1 were identified. A ranR mutant exhibited a phenotype similar to that of an anf1 mutant strain (no growth with N2 in the absence of molybdenum, but significant reduction of acetylene via the Fe-only nitrogenase), whereas a ranT mutant retained the ability to grow diazotrophically, but growth was clearly delayed compared to the parental strain. In contrast to the situation for anf1, expression of neither ranR nor ranT was regulated by ammonium or molybdenum. A putative role for Anf1, RanR, and RanT in the acquisition and/or processing of iron in connection with the Fe-only nitrogenase system is discussed.
Journal of Bacteriology 02/2005; 187(1):92-8. DOI:10.1128/JB.187.1.92-98.2005 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Rhodobacter capsulatus contains two PII-like proteins, GlnB and GlnK, which play central roles in controlling the synthesis and activity of nitrogenase in response to ammonium availability. Here we used the yeast two-hybrid system to probe interactions between these PII-like proteins and proteins known to be involved in regulating nitrogen fixation. Analysis of defined protein pairs demonstrated the following interactions: GlnB-NtrB, GlnB-NifA1, GlnB-NifA2, GlnB-DraT, GlnK-NifA1, GlnK-NifA2, and GlnK-DraT. These results corroborate earlier genetic data and in addition show that PII-dependent ammonium regulation of nitrogen fixation in R. capsulatus does not require additional proteins, like NifL in Klebsiella pneumoniae. In addition, we found interactions for the protein pairs GlnB-GlnB, GlnB-GlnK, NifA1-NifA1, NifA2-NifA2, and NifA1-NifA2, suggesting that fine tuning of the nitrogen fixation process in R. capsulatus may involve the formation of GlnB-GlnK heterotrimers as well as NifA1-NifA2 heterodimers. In order to identify new proteins that interact with GlnB and GlnK, we constructed an R. capsulatus genomic library for use in yeast two-hybrid studies. Screening of this library identified the ATP-dependent helicase PcrA as a new putative protein that interacts with GlnB and the Ras-like protein Era as a new protein that interacts with GlnK.
Journal of Bacteriology 10/2003; 185(17):5240-7. DOI:10.1128/JB.185.17.5240-5247.2003 · 2.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In most bacteria, nitrogen metabolism is tightly regulated and P(II) proteins play a pivotal role in the regulatory processes. Rhodobacter capsulatus possesses two genes (glnB and glnK) encoding P(II)-like proteins. The glnB gene forms part of a glnB-glnA operon and the glnK gene is located immediately upstream of amtB, encoding a (methyl-) ammonium transporter. Expression of glnK is activated by NtrC under nitrogen-limiting conditions. The synthesis and activity of the molybdenum and iron nitrogenases of R. capsulatus are regulated by ammonium on at least three levels, including the transcriptional activation of nifA1, nifA2 and anfA by NtrC, the regulation of NifA and AnfA activity by two different NtrC-independent mechanisms, and the post-translational control of the activity of both nitrogenases by reversible ADP-ribosylation of NifH and AnfH as well as by ADP-ribosylation independent switch-off. Mutational analysis revealed that both P(II)-like proteins are involved in the ammonium regulation of the two nitrogenase systems. A mutation in glnB results in the constitutive expression of nifA and anfA. In addition, the post-translational ammonium inhibition of NifA activity is completely abolished in a glnB-glnK double mutant. However, AnfA activity was still suppressed by ammonium in the glnB-glnK double mutant. Furthermore, the P(II)-like proteins are involved in ammonium control of nitrogenase activity via ADP-ribosylation and the switch-off response. Remarkably, in the glnB-glnK double mutant, all three levels of the ammonium regulation of the molybdenum (but not of the alternative) nitrogenase are completely circumvented, resulting in the synthesis of active molybdenum nitrogenase even in the presence of high concentrations of ammonium.