Function and regulation of glnA in the methanogenic archaeon Methanococcusmaripaludis

The glnA gene in the domains Bacteria and Archaea encodes glutamine synthet ase, a universally distributed enzyme that functions in ammonia assimilatio n and glutamine synthesis. We investigated the regulation and function of g lnA in the methanogenic archaeon Methanococcus maripaludis. The deduced ami no acid sequence of the gene demonstrated its membership in class GSI-alpha of glutamine synthetases. The gene appeared to be expressed as a monocistr onic operon. glnA mRNA levels and specific activities of glutamine syntheta se were regulated similarly by nitrogen. Three transcription start sites we re identified, corresponding to two overlapping nitrogen-regulated promoter s and one weaker constitutive promoter. An inverted repeat immediately upst ream of the regulated transcription start sites mediated repression under n oninducing conditions. Thus, mutations that altered the sequence of the inv erted repeat resulted in derepression. The inverted repeat had sequence sim ilarity, with a repeat that we previously identified as the nif operator of M. maripaludis, suggesting a common mechanism of nitrogen regulation. Effo rts to produce a glnA null mutant failed, suggesting that glnA is an essent ial gene in M. maripaludis.