REGULATION OF NITROGEN-METABOLISM IS ALTERED IN A GLNB MUTANT STRAIN OF RHIZOBIUM-LEGUMINOSARUM

We isolated a Rhizobium leguminosarum mutant strain altered in the gln B gene. This event, which has never been described in the Rhizobiaceae , is rare in comparison to mutants isolated in the contiguous gene, gl nA. The glnB mutation removes the glnBA promoter but in vivo does not prevent glnA expression from its own promoter, which is not nitrogen r egulated. The glnB mutant strain does not grow on nitrate as a sole ni trogen source and it is Nod(+), Fix(+). Two -24/-12 promoters, for the glnII and glnSA genes, are constitutively expressed in the glnB mutan t, while two -35/-10-like promoters for glnA and ntrBC are unaffected. We propose that the glnB gene product, the P-II protein, plays a nega tive role in the ability of NtrC to activate transcription from its ta rget promoters and a positive role in the mechanism of nitrate utiliza tion.