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

Abstract

The fixation of atmospheric nitrogen by the prokaryotic enzyme nitrogenase is an energyexpensive 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. IntroductionBiological nitrogen fixation, the reduction of atmospheric N 2 to NH 3 by nitrogen-fixing bacteria, is a key step in the nitrogen cycle. This process is catalysed by nitrogenase, the most common form of which is the molybdenum nitrogenase, composed of dinitrogenase (MoFe protein or NifDK), an a 2 b 2 tetramer encoded by the nifD and nifK genes, respectively, and dinitrogenase reductase (Fe protein or NifH), a c 2 homodimer encoded by the nifH gene. The NifH protein is responsible for ATP-hydrolysisdriven electron transport to the NifDK protein, which contains the site for the reduction of dinitrogen to ammonium (Seefeldt et al., 2009). The reduction of N 2 to two molecules of ammonium is an energy-expensive process requiring the hydrolysis of 16 ATPs.To avoid energy wastage, diazotrophs have evolved both transcriptional and post-translational mechanisms to shut-down nitrogen fixation when ammonium is available in the environment. Post-translational control of nitrogenase activity has been found in a range of diazotrophs and affords a rapid and reversible mechanism by which the organism can respond to transient changes in the environment. Here we review current knowledge of the different mechanisms of post-translational control of nitrogenase. We focus on the two best-described systems: ADP-ribosylation of NifH, which occurs in proteobacteria, and the interaction of NifI regulatory proteins with NifDK in archaea, which potentially also operates in some anaerobic diazotrophic bacteria. Regulation of nitrogenase activity by reversible ADP-ribosylation Historical perspectiveThe process of metabolic inactivation of nitrogenase in response to ammonium was first described in Azotobacter vinelandii (Burris & Wilson, 1946); this phenomenon was later identified in other prokaryotes and named nitrogenase 'switch-off' (Zumft & Castillo, 1978). Different mechanisms are used to regulate nitrogenase post-translationally depending on the organism. The best-studied mechanism operates through reversible ADP-ribosylation of NifH (reviewed by Nordlund, 2000). This system responds to not only the presence of ammonium but also a decrease in the availability of cellular energy, in response to either darkness in phototrophs such as ...

The Emergence of 2-Oxoglutarate as a Master Regulator Metabolite

“…P II proteins are well conserved and widely distributed in bacteria, in archaea, and in the chloroplasts of plants (35). Since this protein family was recently reviewed comprehensively (36)(37)(38), in this review we briefly describe the P II proteins and then focus on the regulation of acetyl-CoA carboxylase (ACC) by P II .…”

“…The Nitrogenase Transcriptional Regulator Protein NifA P II proteins are involved in the regulation of nitrogen fixation in both archaea and bacteria, affecting the transcription of nitrogen fixation genes (nif genes) and/or nitrogenase activity (36,53,62). In proteobacteria, P II influences nif gene expression by controlling the activity of a transcriptional activator named NifA in response (Q39) residues (green sticks) and also with MgATP.…”

The Nitrogenase Regulatory Enzyme Dinitrogenase Reductase ADP-Ribosyltransferase (DraT) Is Activated by Direct Interaction with the Signal Transduction Protein GlnB

“…This mechanism has been extensively studied in the alphaproteobacteria Azospirillum brasilense and Rhodospirillum rubrum. Furthermore, bioinformatics analysis supports the idea that a similar mechanism operates in at least 25 bacterial genera (2). Fe protein ADP-ribosylation is catalyzed by dinitrogenase reductase ADP-ribosyltransferase (DraT).…”

“…In response to a negative stimulus, such as the presence of ammonium or a decrease in the available cell energy, DraT is activated and DraG is inactivated, promoting inactivation of nitrogenase by ADP-ribosylation of the Fe protein. Conversely, when the negative stimulus is removed, DraT is inactivated and DraG is activated, favoring ADP-ribose removal from the Fe protein and nitrogenase activity (2,7). The signaling pathway linking the cellular nitrogen status to regulated activities of DraT and DraG in A. brasilense and R. rubrum has been extensively studied and relies on the nitrogen signal transduction proteins of the P II protein family.…”

“…The MoFe protein contains the N 2 binding/reduction active site (1). In order to avoid wasteful ATP hydrolysis, several microorganisms have acquired diverse mechanisms to inactivate nitrogenase at the posttranslational level when fixed nitrogen (e.g., ammonium) is available in the environment (2).…”