Genetics of nitrogen regulation in Methanococcus maripaludis

We have used genetic methods in Methanococcus maripaludis to study nitrogen metabolism and its regulation. We present evidence for a "nitrogen regulon " in Methanococcus and Methanobacterium species containing genes of nitroge n metabolism that are regulated coordinately at the transcriptional level v ia a common repressor binding site sequence, or operator. The implied mecha nism for regulation resembles the general bacterial paradigm for repression , but contrasts with well-known mechanisms of nitrogen regulation in bacter ia, which occur by activation. Genes in the nitrogen regulons include those for nitrogen fixation, glutamine synthetase, (methyl)ammonia transport, th e regulatory protein GlnB, and ammonia-dependent NAD synthetase, as well as a gene of unknown function. We also studied the function of mio novel GlnB homologues that are encoded within the nif gene cluster of diazotrophic me thanogens. The phenotype resulting from a glnB null mutation in M. maripalu dis provides direct evidence that glnB-like genes are involved in "ammonia switch-off," the post-transcriptional inhibition of nitrogen fixation upon addition of ammonia. Finally, we show that the gene nifX is not required fo r nitrogen fixation, in agreement with findings in several bacteria. These studies illustrate the utility of genetic methods in M, maripaludis and sho w the enhanced perspective that studies in the Archaea can bring to known b iological systems.