Ammonia switch-off of nitrogen fixation in the methanogenic archaeon Methanococcus maripaludis: Mechanistic features and requirement for the novel GLnB homologues, NifI(1) and NifI(2)

Ammonia switch-off is the immediate inactivation of nitrogen fixation that occurs when a superior nitrogen source is encountered. In certain bacteria switch-off occurs by reversible covalent ADP-ribosylation of the dinitrogen ase reductase protein, NifH. Ammonia switch-off occurs in diazotrophic spec ies of the methanogenic Archaea as well. We showed previously that in Metha nococcus maripaludis switch-off requires at least one of two novel homologu es of glnB, a family of genes whose products play a central role in nitroge n sensing and regulation in bacteria. The novel glnB homologues have recent ly been named nifI(1) and nifI(2). Here we use in-frame deletions and genet ic complementation analysis in M. maripaludis to show that the nifI(1) and nifI(2) genes are both required for switch-off. We could not detect ADP-rib osylation or any other covalent modification of dinitrogenase reductase dur ing switch-off, suggesting that the mechanism differs from the well-studied bacterial system. Furthermore, switch-off did not affect nif gene transcri ption, nifH mRNA stability, or NifH protein stability. Nitrogenase activity resumed within a short time after ammonia was removed from a switched-off culture, suggesting that whatever the mechanism, it is reversible, We demon strate the physiological importance of switch off by showing that it allows growth to accelerate substantially when a diazotrophic culture is switched to ammonia.