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

PII signal transduction proteins: pivotal players in post-translational control of nitrogenase activity

The fixation of atmospheric nitrogen by the prokaryotic enzyme nitrogenase is an energy- expensive process and consequently it is tightly regulated at a variety of levels. In many diazotrophs this includes post-translational regulation of the enzyme's activity, which has been reported in both bacteria and archaea. The best understood response is the short-term inactivation of nitrogenase in response to a transient rise in ammonium levels in the environment. A number of proteobacteria species effect this regulation through reversible ADP-ribosylation of the enzyme, but other prokaryotes have evolved different mechanisms. Here we review current knowledge of post-translational control of nitrogenase and show that, for the response to ammonium, the P(II) signal transduction proteins act as key players.

Mechanism of ADP-ribosylation removal revealed by the structure and ligand complexes of the dimanganese mono-ADP-ribosylhydrolase DraG

ADP-ribosylation is a ubiquitous regulatory posttranslational modification involved in numerous key processes such as DNA repair, transcription, cell differentiation, apoptosis, and the pathogenic mechanism of certain bacterial toxins. Despite the importance of this reversible process, very little is known about the structure and mechanism of the hydrolases that catalyze removal of the ADP-ribose moiety. In the phototrophic bacterium Rhodospirillum rubrum, dinitrogenase reductase-activating glycohydrolase (DraG), a dimanganese enzyme that reversibly associates with the cell membrane, is a key player in the regulation of nitrogenase activity. DraG has long served as a model protein for ADP-ribosylhydrolases. Here, we present the crystal structure of DraG in the holo and ADP-ribose bound forms. We also present the structure of a reaction intermediate analogue and propose a detailed catalytic mechanism for protein de-ADP-ribosylation involving ring opening of the substrate ribose. In addition, the particular manganese coordination in DraG suggests a rationale for the enzyme's preference for manganese over magnesium, although not requiring a redox active metal for the reaction.

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.

Effect of AmtB homologues on the post-translational regulation of nitrogenase activity in response to ammonium and energy signals in Rhodospirillum rubrum

The AmtB protein transports uncharged NH(3) into the cell, but it also interacts with the nitrogen regulatory protein P(II), which in turn regulates a variety of proteins involved in nitrogen fixation and utilization. Three P(II) homologues, GlnB, GlnK and GlnJ, have been identified in the photosynthetic bacterium Rhodospirillum rubrum, and they have roles in at least four overlapping and distinct functions, one of which is the post-translational regulation of nitrogenase activity. In R. rubrum, nitrogenase activity is tightly regulated in response to addition or energy depletion (shift to darkness), and this regulation is catalysed by the post-translational regulatory system encoded by draTG. Two amtB homologues, amtB(1) and amtB(2), have been identified in R. rubrum, and they are linked with glnJ and glnK, respectively. Mutants lacking AmtB(1) are defective in their response to both addition and darkness, while mutants lacking AmtB(2) show little effect on the regulation of nitrogenase activity. These responses to darkness and appear to involve different signal transduction pathways, and the poor response to darkness does not seem to be an indirect result of perturbation of internal pools of nitrogen. It is also shown that AmtB(1) is necessary to sequester detectable amounts GlnJ to the cell membrane. These results suggest that some element of the AmtB(1)-P(II) regulatory system senses energy deprivation and a consistent model for the integration of nitrogen, carbon and energy signals by P(II) is proposed. Other results demonstrate a degree of specificity in interaction of AmtB(1) with the different P(II) homologues in R. rubrum. Such interaction specificity might be important in explaining the way in which P(II) proteins regulate processes involved in nitrogen acquisition and utilization.

ADP-ribosylation of dinitrogenase reductase in Azospirillum brasilense is regulated by AmtB-dependent membrane sequestration of DraG

Nitrogen fixation in some diazotrophic bacteria is regulated by mono-ADP-ribosylation of dinitrogenase reductase (NifH) that occurs in response to addition of ammonium to the extracellular medium. This process is mediated by dinitrogenase reductase ADP-ribosyltransferase (DraT) and reversed by dinitrogenase reductase glycohydrolase (DraG), but the means by which the activities of these enzymes are regulated are unknown. We have investigated the role of the P(II) proteins (GlnB and GlnZ), the ammonia channel protein AmtB and the cellular localization of DraG in the regulation of the NifH-modification process in Azospirillum brasilense. GlnB, GlnZ and DraG were all membrane-associated after an ammonium shock, and both this membrane sequestration and ADP-ribosylation of NifH were defective in an amtB mutant. We now propose a model in which membrane association of DraG after an ammonium shock creates a physical separation from its cytoplasmic substrate NifH thereby inhibiting ADP-ribosyl-removal. Our observations identify a novel role for an ammonia channel (Amt) protein in the regulation of bacterial nitrogen metabolism by mediating membrane sequestration of a protein other than a P(II) family member. They also suggest a model for control of ADP-ribosylation that is likely to be applicable to all diazotrophs that exhibit such post-translational regulation of nitrogenase.

Metabolic regulation of nitrogen fixation in Rhodospirillum rubrum

Nitrogenase activity in Rhodospirillum rubrum is post-translationally regulated by DRAG (dinitrogenase reductase glycohydrolase) and DRAT (dinitrogenase reductase ADP-ribosylation transferase). When a sudden increase in fixed nitrogen concentration or energy depletion is sensed by the cells, DRAG is inactivated and DRAT activated. We propose that the regulation of DRAG is dependent on its location in the cell and the presence of an ammonium-sensing protein.

Reversible membrane association of dinitrogenase reductase activating glycohydrolase in the regulation of nitrogenase activity in Rhodospirillum rubrum; dependence on GlnJ and AmtB1

In the photosynthetic bacterium Rhodospirillum rubrum nitrogenase activity is regulated by reversible ADP-ribosylation of dinitrogenase reductase in response to external so called "switch-off" effectors. Activation of the modified, inactive form is catalyzed by dinitrogenase reductase activating glycohydrolase (DRAG) which removes the ADP-ribose moiety. This study addresses the signal transduction between external effectors and DRAG. R. rubrum, wild-type and P(II) mutant strains, were studied with respect to DRAG localization. We conclude that GlnJ clearly has an effect on the association of DRAG to the membrane in agreement with the effect on regulation of nitrogenase activity. Furthermore, we have generated a R. rubrum mutant lacking the putative ammonium transporter AmtB1 which was shown not to respond to "switch-off" effectors; no loss of nitrogenase activity and no ADP-ribosylation. Interestingly, DRAG was mainly localized to the cytosol in this mutant. Overall the results support our model in which association to the membrane is part of the mechanism regulating DRAG activity.

Characterization of altered regulation variants of dinitrogenase reductase-activating glycohydrolase from Rhodospirillum rubrum

In Rhodospirillum rubrum, nitrogenase activity is subject to posttranslational regulation through the adenosine diphosphate (ADP)-ribosylation of dinitrogenase reductase by dinitrogenase reductase ADP-ribosyltransferase (DRAT) and dinitrogenase reductase-activating glycohydrolase (DRAG). To study the posttranslational regulation of DRAG, its gene was mutagenized and colonies screened for altered DRAG regulation. Three different mutants were found and the DRAG variants displayed different biochemical properties including an altered affinity for divalent metal ions. Taken together, the results suggest that the site involved in regulation is physically near the metal binding site of DRAG.

Regulation of dinitrogenase reductase ADP-ribosyltransferase and dinitrogenase reductase-activating glycohydrolase by a redox-dependent conformational change of nitrogenase Fe protein

The nitrogenase-regulating enzymes dinitrogenase reductase ADP-ribosyltransferase (DRAT) and dinitrogenase reductase-activating glycohydrolase (DRAG), from Rhodospirillum rubrum, were shown to be sensitive to the redox status of the [Fe(4)S(4)](1+/2+) cluster of nitrogenase Fe protein from R. rubrum or Azotobacter vinelandii. DRAG had <2% activity with oxidized R. rubrum Fe protein relative to activity with reduced Fe protein. The activity of DRAG with oxygen-denatured Fe protein or a low molecular weight substrate, N(alpha)-dansyl-N(omega)-(1,N(6)-etheno-ADP-ribosyl)-arginine methyl ester, was independent of redox potential. The redox midpoint potential of DRAG activation of Fe protein was -430 mV versus standard hydrogen electrode, coinciding with the midpoint potential of the [Fe(4)S(4)] cluster from R. rubrum Fe protein. DRAT was found to have a specificity opposite that of DRAG, exhibiting low (<20%) activity with 87% reduced R. rubrum Fe protein relative to activity with fully oxidized Fe protein. A mutant of R. rubrum in which the rate of oxidation of Fe protein was substantially decreased had a markedly slower rate of ADP-ribosylation in vivo in response to 10 mM NH(4)Cl or darkness stimulus. It is concluded that the redox state of Fe protein plays a significant role in regulation of the activities of DRAT and DRAG in vivo.