A novel regulatory role of the Rnf complex of Azoarcus sp. strain BH72

Summary The superfamily of PII proteins contains the most widely distributed signalling proteins in nature. Remarkable is the variety of targets whose activity is affected by protein–protein interactions. Here we identified as novel partner for interaction with GlnK an Rnf complex, known to couple the energy of ion transport to reduce ferredoxins. The endophytic diazotrophic betaproteobacterium Azoarcus sp. strain BH72 harbours two rnf‐like clusters in the genome, of which only the rnf1 cluster was induced under conditions of N2 fixation under control of the transcriptional activator NifA. Rapid inactivation (‘DraT‐independent switch off’) of nitrogenase activity upon ammonium upshift was dependent on the Rnf1 complex. Membrane sequestration of GlnK in steady‐state N‐surplus conditions occurred in its unmodified form, signalling N‐surplus, and was dependent on presence of the Rnf1 complex, suggesting physical interaction. In vitro binding studies by Far‐Western analysis indicated interactions of RnfC1 with specifically GlnK but not with GlnB. As ammonium upshift led to decreased activity of the Rnf1 complex in membranes, it might be inactivated by GlnK binding, leading to an interruption of electron flow to nitrogenase and thus a rapid, DraT‐independent nitrogenase switch off. Our data imply a hitherto unknown interaction partner for a PII‐like protein and an additional process under its control.