Studies on the effect of NAD(H) on nitrogenase activity in Rhodospirillum rubrum

Posttranslational modification of dinitrogenase reductase in Rhodospirillum rubrum treated with fluoroacetate

Nitrogen fixation is one of the major biogeochemical contributions carried out by diazotrophic microorganisms. The goal of this research is study of posttranslational modification of dinitrogenase reductase (Fe protein), the involvement of malate and pyruvate in generation of reductant in Rhodospirillum rubrum. A procedure for the isolation of the Fe protein from cell extracts was developed and used to monitor the modification of the Fe protein in vivo. The subunit pattern of the isolated the Fe protein after sodium dodecyl sulfate-polyacrylamide gel electrophoresis was assayed by Western blot analysis. Whole-cell nitrogenase activity was also monitored during the Fe protein modification by gas chromatograpy, using the acetylene reduction assay. It has been shown, that the addition of fluoroacetate, ammonia and darkness resulted in the loss of whole-cell nitrogenase activity and the in vivo modification of the Fe protein. For fluoroacetate, ammonia and darkness, the rate of loss of nitrogenase activity was similar to that for the Fe protein modification. The addition of NADH and reillumination of a culture incubated in the dark resulted in the rapid restoration of nitrogenase activity and the demodification of the Fe protein. Fluoroacetate inhibited the nitrogenase activity of R. rubrum and resulted in the modification of the Fe protein in cells, grown on pyruvate or malate as the endogeneous electron source. The nitrogenase activity in draTG mutant (lacking DRAT/DRAG system) decreased after the addition of fluoroacetate, but the Fe protein remained completely unmodified. The results showed that the reduced state of cell, posttranslational modifications of the Fe protein and the DRAT/DRAG system are important for nitrogenase activity and the regulation of nitrogen fixation.

Formation of a cross-linking complex of dinitrogenase reductase-activating glycohydrolase (DRAG) with membrane proteins from Rhodospirillum rubrum chromatophores

Association of dinitrogenase reductase-activating glycohydrolase (DRAG) with membrane proteins of chromatophores has been investigated. The formation of a multicomponent complex between DRAG and membrane proteins was demonstrated in the presence of glutaraldehyde and EDC/NHS (N-(3-dimethylaminopropyl)-N -ethylcarbodiimide hydrochloride/hydroxy-2,5-dioxopyrrolidine-3-sulfonic acid sodium salt). Complex formation was observed both in native chromatophore membrane and in chromatophores treated with 0.5 M NaCl in the presence of homogeneous DRAG and glutaraldehyde in cross-reaction. The molecular weight of the complex was around 200 kD, which is consistent with the association of DRAG with three or more chromatophore membrane proteins. A specific complex with molecular weight of about 75 kD was formed only in the presence of EDC/NHS in the cross-linking reaction. It was demonstrated that ammonium transport protein and P11 protein are possible candidates for association with DRAG in chromatophore membranes.

NAD-, NMN-, and NADP-dependent modification of dinitrogenase reductases from Rhodospirillum rubrum and Azotobacter vinelandii

Nitrogenase activity in the photosynthetic bacterium Rhodospirillum rubrum is reversibly regulated by ADP-ribosylation of a specific arginine residue of dinitrogenase reductase based on the cellular nitrogen or energy status. In this paper, we have investigated the ability of nicotinamide adenine dinucleotide, NAD (the physiological ADP-ribose donor), and its analogs to support covalent modification of dinitrogenase reductase in vitro. R. rubrum dinitrogenase reductase can be modified by DRAT in the presence of 2 mM NAD, but not with 2 mM nicotinamide mononucleotide (NMN) or nicotinamide adenine dinucleotide phosphate (NADP). We also found that the apo- and the all-ferrous forms of R. rubrum dinitrogenase reductase are not substrates for covalent modification. In contrast, Azotobacter vinelandii dinitrogenase reductase can be modified by the dinitrogenase reductase ADP-ribosyl transferase (DRAT) in vitro in the presence of either 2 mM NAD, NMN or NADP as nucleotide donors. We found that: (1) a simple ribose sugar in the modification site of the A. vinelandii dinitrogenase reductase is sufficient to inactivate the enzyme, (2) phosphoADP-ribose is the modifying unit in the NADP-modified enzyme, and (3) the NMN-modified enzyme carries two ribose-phosphate units in one modification site. This is the first report of NADP- or NMN-dependent modification of a target protein by an ADP-ribosyl transferase.

Functional characterization of three GlnB homologs in the photosynthetic bacterium Rhodospirillum rubrum: roles in sensing ammonium and energy status

The GlnB (P(II)) protein, the product of glnB, has been characterized previously in the photosynthetic bacterium Rhodospirillum rubrum. Here we describe identification of two other P(II) homologs in this organism, GlnK and GlnJ. Although the sequences of these three homologs are very similar, the molecules have both distinct and overlapping functions in the cell. While GlnB is required for activation of NifA activity in R. rubrum, GlnK and GlnJ do not appear to be involved in this process. In contrast, either GlnB or GlnJ can serve as a critical element in regulation of the reversible ADP ribosylation of dinitrogenase reductase catalyzed by the dinitrogenase reductase ADP-ribosyl transferase (DRAT)/dinitrogenase reductase-activating glycohydrolase (DRAG) regulatory system. Similarly, either GlnB or GlnJ is necessary for normal growth on a variety of minimal and rich media, and any of the proteins is sufficient for normal posttranslational regulation of glutamine synthetase. Surprisingly, in their regulation of the DRAT/DRAG system, GlnB and GlnJ appeared to be responsive not only to changes in nitrogen status but also to changes in energy status, revealing a new role for this family of regulators in central metabolic regulation.

P(II) signal transduction proteins, pivotal players in microbial nitrogen control

The P(II) family of signal transduction proteins are among the most widely distributed signal proteins in the bacterial world. First identified in 1969 as a component of the glutamine synthetase regulatory apparatus, P(II) proteins have since been recognized as playing a pivotal role in control of prokaryotic nitrogen metabolism. More recently, members of the family have been found in higher plants, where they also potentially play a role in nitrogen control. The P(II) proteins can function in the regulation of both gene transcription, by modulating the activity of regulatory proteins, and the catalytic activity of enzymes involved in nitrogen metabolism. There is also emerging evidence that they may regulate the activity of proteins required for transport of nitrogen compounds into the cell. In this review we discuss the history of the P(II) proteins, their structures and biochemistry, and their distribution and functions in prokaryotes. We survey data emerging from bacterial genome sequences and consider other likely or potential targets for control by P(II) proteins.

Isolation and characterization of draT mutants that have altered regulatory properties of dinitrogenase reductase ADP-ribosyltransferase in Rhodospirillum rubrum

In Rhodospirillum rubrum, dinitrogenase reductase ADP-ribosyltransferase (DRAT) is responsible for the ADP-ribosylation of dinitrogenase reductase in response to the addition of NH(+)(4) or removal from light, resulting in a decrease in nitrogenase activity. DRAT is itself subject to post-translational regulation; to investigate the mechanism for the regulation of DRAT activity, random PCR mutagenesis of draT (encoding DRAT) was performed and mutants with altered DRAT regulation were screened. Two mutants (with substitutions of K103E and N248D) were obtained in which DRAT showed activity under conditions where wild-type DRAT (DRAT-WT) did not. These mutants showed lower nitrogenase activity and a higher degree of ADP-ribosylation of dinitrogenase reductase under N(2)-fixing conditions than was seen in a wild-type control strain. DRAT-K103E was overexpressed and purified. DRAT-K103E displayed a much weaker affinity for an Affi-gel Blue matrix than did DRAT-WT, suggestive of a fairly striking biochemical change. However, there was no significant difference in kinetic constants, such as K(m) for NAD and V(max), between DRAT-K103E and DRAT-WT. Like DRAT-WT, DRAT-K103E also modified reduced dinitrogenase reductase poorly. The biochemical properties of these variants are rationalized with respect to their behaviour in vivo.

Dinitrogenase reductase-activating glycohydrolase can be released from chromatophores of Rhodospirillum rubrum by treatment with MgGDP

Dinitrogenase reductase-activating glycohydrolase (DRAG), involved in the regulation of nitrogenase activity in Rhodospirillum rubrum, is associated with chromatophore membranes in cell extracts. We show that DRAG can be specifically released by treatment with MgGDP; other nucleotides studied had no effect. The DRAG activity released corresponds to the release of DRAG protein.

Regulation of nitrogen fixation in Azospirillum brasilense

The regulation of nitrogen fixation in Azospirillum brasilense is very complicated, and it responds to exogenous fixed nitrogen or a change of oxygen concentration. This regulation occurs at both transcriptional and posttranslational levels. Unlike regulation seen in Klebsiella pneumoniae, transcription of nifA does not require NTRB/NTRC in A. brasilense and the expression of nifHDK is controlled by posttranslational regulation of NIFA activity. Addition of NH4+ or a shift from microaerobic to anaerobic conditions also causes a rapid loss of nitrogenase activity in A. brasilense. This posttranslational regulation of nitrogenase activity involves the DRAT/DRAG regulatory system, which is similar to that of Rhodospirillum rubrum. Both DRAT and DRAG activities are regulated in vivo, but the mechanisms for their regulation are unknown.

Uridylylation of the P(II) protein in the photosynthetic bacterium Rhodospirillum rubrum

The regulatory protein P(II) has been studied in great detail in enteric bacteria; however, its function in photosynthetic bacteria has not been clearly established. As a number of these bacteria have been shown to regulate nitrogenase activity by a metabolic control system, it is of special interest to establish the role of P(II) in these diazotrophs. In this study, we show that P(II) in Rhodospirillum rubrum is modified in response to the N status in the cell and that addition of ammonium or glutamine leads to demodification. We also provide evidence that P(II) is uridylylated. In addition, we show that not only these compounds but also NAD+ promotes demodification of P(II), which is of particular interest as this pyridine nucleotide has been shown to act as a switch-off effector of nitrogenase. Demodification of P(II) by ammonium or NAD+ did not occur in cultures treated with an inhibitor of glutamine synthetase (methionine sulfoximine), whereas treatment with the glutamate synthase inhibitor 6-diazo-5-oxo-norleucine led to total demodification of P(II) without any other addition. The results indicate that P(II) probably is not directly involved in darkness switch-off of nitrogenase but that a role in ammonium switch-off cannot be excluded.

NAD-dependent cross-linking of dinitrogenase reductase and dinitrogenase reductase ADP-ribosyltransferase from Rhodospirillum rubrum

Chemical cross-linking of dinitrogenase reductase and dinitrogenase reductase ADP-ribosyltransferase (DRAT) from Rhodospirillum rubrum has been investigated with a cross-linking system utilizing two reagents, 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide and sulfo-N-hydroxysuccinimide. Cross-linking between dinitrogenase reductase and DRAT requires the presence of NAD, the cellular ADP-ribose donor, or a NAD analog containing an unmodified nicotinamide group, such as nicotinamide hypoxanthine dinucleotide. NADP, which will not replace NAD in the modification reaction, does support cross-linking between dinitrogenase reductase and DRAT. The DRAT-catalyzed ADP-ribosylation of dinitrogenase reductase is inhibited by sodium chloride, as is the cross-linking between dinitrogenase reductase and DRAT, suggesting that ionic interactions are required for the association of these two proteins. Cross-linking is specific for native, unmodified dinitrogenase reductase, in that both oxygen-denatured and ADP-ribosylated dinitrogenase reductase fail to form a cross-linked complex with DRAT. The ADP-bound and adenine nucleotide-free states of dinitrogenase reductase form cross-linked complexes with DRAT; however, cross-linking is inhibited when dinitrogenase reductase is in its ATP-bound state.

The role of NAD+ as a signal during nitrogenase switch-off in Rhodospirillum rubrum

The role of NAD+ in the metabolic regulation of nitrogenase, the 'switch-off' effect, in Rhodospirillum rubrum has been studied. We now show that the decrease in nitrogenase activity upon addition of NAD+ to R. rubrum is due to modification of dinitrogenase reductase. There was no effect when NAD+ was added to a mutant of R. rubrum devoid of dinitrogenase reductase ADP-ribosyltransferase, indicating that NAD+ 'switch-off' is an effect of the same regulatory system as ammonium 'switch-off'. We also show that oxaloacetate and alpha-ketoglutarate function as 'switch-off' effectors. On the other hand beta-hydroxybutyrate has the opposite effect by shortening the 'switch-off' period. Furthermore, by using an inhibitor of glutamate synthase the role of this enzyme in 'switch-off' was investigated. The results are discussed in relation to our proposal that changes in the concentration of NAD+ are involved in initiating 'switch-off'.

Comparison studies of dinitrogenase reductase ADP-ribosyl transferase/dinitrogenase reductase activating glycohydrolase regulatory systems in Rhodospirillum rubrum and Azospirillum brasilense

Reversible ADP ribosylation of dinitrogenase reductase, catalyzed by the dinitrogenase reductase ADP-ribosyl transferase (DRAT)/dinitrogenase reductase activating glycohydrolase (DRAG) regulatory system, has been characterized in both Rhodospirillum rubrum and Azospirillum brasilense. Although the general functions of DRAT and DRAG are very similar in these two organisms, there are a number of interesting differences, e.g., in the timing and extent of the regulatory response to different stimuli. In this work, the basis of these differences has been studied by the heterologous expression of either draTG or nifH from A. brasilense in R. rubrum mutants that lack these genes, as well as the expression of draTG from R. rubrum in an A. brasilense draTG mutant. In general, these hybrid strains respond to stimuli in a manner similar to that of the wild-type parent of the recipient strain rather than the wild-type source of the introduced genes. These results suggest that the differences seen in the regulatory response in these organisms are not primarily a result of different properties of DRAT, DRAG, or dinitrogenase reductase. Instead, the differences are likely the result of different signal pathways that regulate DRAG and DRAT activities in these two organisms. Our results also suggest that draT and draG are cotranscribed in A. brasilense.

Changes in the NAD(P)H concentration caused by addition of nitrogenase 'switch-off' effectors in Rhodospirillum rubrum G-9, as measured by fluorescence

The effect of nitrogenase 'switch-off' effectors on the concentration of NAD(P)H in Rhodospirillum rubrum G-9 was investigated by fluorescence. A rapid decrease in fluorescence was observed when cells, either N2-grown or nitrogen-starved, were subjected to the effectors, but not when sodium chloride or Tris buffer was added. No effects on the fluorescence were observed in non-nitrogen fixing cultures except when NAD+ was added. The results strongly indicate that the redox state of the pyridine nucleotide pool affects the control of the regulation of nitrogenase activity in R. rubrum.

Posttranslational regulation of nitrogenase activity in Azospirillum brasilense ntrBC mutants: ammonium and anaerobic switch-off occurs through independent signal transduction pathways

Nitrogenase activity is regulated by reversible ADP-ribosylation in response to NH4+ and anaerobic conditions in Azospirillum brasilense. The effect of mutations in ntrBC on this regulation was examined. While NH4+ addition to ntrBC mutants caused a partial loss of nitrogenase activity, the effect was substantially smaller than that seen in ntr+ strains. In contrast, nitrogenase activity in these mutants was normally regulated in response to anaerobic conditions. The analysis of mutants lacking both the ntrBC gene products and dinitrogenase reductase activating glycohydrolase (DRAG) suggested that the primary effect of the ntrBC mutations was to alter the regulation of DRAG activity. Although nif expression in the ntr mutants appeared normal, as judged by activity, glutamine synthetase activity was significantly lower in ntrBC mutants than in the wild type. We hypothesize that this lower glutamine synthetase activity may delay the transduction of the NH4+ signal necessary for the inactivation of DRAG, resulting in a reduced response of nitrogenase activity to NH4+. Finally, data presented here suggest that different environmental stimuli use independent signal pathways to affect this reversible ADP-ribosylation system.

Posttranslational regulation of nitrogenase activity by anaerobiosis and ammonium in Azospirillum brasilense

In the microaerophilic diazotroph Azospirillum brasilense, the addition of fixed nitrogen or a shift to anaerobic conditions leads to a rapid loss of nitrogenase activity due to ADP-ribosylation of dinitrogenase reductase. The product of draT (DRAT) is shown to be necessary for this modification, and the product of draG (DRAG) is shown to be necessary for the removal of the modification upon removal of the stimulus. DRAG and DRAT are themselves subject to posttranslational regulation, and this report identifies features of that regulation. We demonstrate that the activation of DRAT in response to an anaerobic shift is transient but that the duration of DRAT activation in response to added NH4+ varies with the NH4+ concentration. In contrast, DRAG appears to be continuously active under conditions favoring nitrogen fixation. Thus, the activities of DRAG and DRAT are not always coordinately regulated. Finally, our experiments suggest the existence of a temporary period of futile cycling during which DRAT and DRAG are simultaneously adding and removing ADP-ribose from dinitrogenase reductase, immediately following the addition of a negative stimulus.