THE RHIZOBIUM-MELILOTI P-II PROTEIN, WHICH CONTROLS BACTERIAL NITROGEN-METABOLISM, AFFECTS ALFALFA NODULE DEVELOPMENT

Symbiotic nitrogen fixation involves the development of specialized or gans called nodules within which plant photosynthates are exchanged fo r combined nitrogen of bacterial origin. To determine the importance o f bacterial nitrogen metabolism in symbiosis, we have characterized a key regulator of this metabolism in Rhizobium meliloti, the uridylylat able P-II protein encoded by glnB. We have constructed both a glnB nul l mutant and a point mutant making nonuridylylatable P-II. In free-liv ing conditions, P-II is required for expression of the ntrC-dependent gene glnII and for adenylylation of glutamine synthetase I. P-II is al so required for efficient infection of alfalfa but not for expression of nitrogenase. However alfalfa plants inoculated with either gLnB mut ant are nitrogen-starved in the absence of added combined nitrogen. We hypothesize that P-II controls expression or activity of a bacteroid ammonium transporter required for a functional nitrogen-fixing symbios is. Therefore, the P-II protein affects both Rhizobium nitrogen metabo lism and alfalfa nodule development.