Nitrogenase catalyzes the conversion of dinitrogen gas (N(2)) and protons to ammonia and hydrogen gas (H(2)). This is a catalytically difficult reaction that requires large amounts of ATP and reducing power. Thus, nitrogenase is not normally expressed or active in bacteria grown with a readily utilized nitrogen source like ammonium. nifA* mutants of the purple nonsulfur phototrophic bacterium Rhodopseudomonas palustris have been described that express nitrogenase genes constitutively and produce H(2) when grown with ammonium as a nitrogen source. This raised the regulatory paradox of why these mutants are apparently resistant to a known posttranslational modification system that should switch off the activity of nitrogenase. Microarray, mutation analysis, and gene expression studies showed that posttranslational regulation of nitrogenase activity in R. palustris depends on two proteins: DraT2, an ADP-ribosyltransferase, and GlnK2, an NtrC-regulated P(II) protein. GlnK2 was not well expressed in ammonium-grown NifA* cells and thus not available to activate the DraT2 nitrogenase modification enzyme. In addition, the NifA* strain had elevated nitrogenase activity due to overexpression of the nif genes, and this increased amount of expression overwhelmed a basal level of activity of DraT2 in ammonium-grown cells. Thus, insufficient levels of both GlnK2 and DraT2 allow H(2) production by an nifA* mutant grown with ammonium. Inactivation of the nitrogenase posttranslational modification system by mutation of draT2 resulted in increased H(2) production by ammonium-grown NifA* cells.