OXYGEN REGULATION OF EXPRESSION OF NITROGEN-FIXATION GENES IN RHIZOBIUM-MELILOTI

The sensor kinase FixL and the response regulator FixJ induce the expr ession of the nitrogen fixation genes of Rhizobium meliloti in respons e to microaerobiosis, which is a characteristic feature of the plant r oot nodule interior where the bacteria fix nitrogen. The kinase activi ty of a purified, soluble derivative of the membrane-bound hemoprotein FixL, designated FixL, is stimulated under low oxygen conditions, th us increasing FixJ-phosphate levels. FixJ-phosphate is a potent transc riptional activator of the nifA and fixK genes, the products of which, in turn, induce the expression of most if not all of the remaining ni trogen fixation genes. FixL and FixL*-phosphate also dephosphorylate FixJ-phosphate, and this activity is depressed by low oxygen concentra tions. In the current model, gene expression is reciprocally coordinat ed by the kinase and phosphatase activities of FixL according to chang es in oxygen tension.