Autophosphorylation and phosphatase activities of the oxygen-sensing protein FixL of Rhizobium meliloti are coordinately regulated by oxygen.

The FixL and FixJ proteins of Rhizobium meliloti control the expression of other nif and fix genes in response to oxygen levels. FixL is a hemoprotein kinase that senses oxygen availability and responds to the absence of oxygen by activation of its autophosphorylating activity followed by transfer of the phosphate to FixJ. FixJ in turn activates the nifA and fixK promoters. In vitro studies reported here with a soluble truncated version of FixL (FixL*) indicate that, while low oxygen tension specifically increases the autophosphorylating activity of FixL*, the ability of phospho-FixL* to act as a phosphate donor to FixJ is not affected by the presence or absence of oxygen. FixL* is also shown to possess a phosphatase activity that is repressed under anaerobic conditions only when the protein is in the phosphorylated form. A fixL mutant that induces a higher level of nifA promoter activity in the presence of fixJ in vivo displayed both an increased autophosphorylating activity and a decreased phosphatase activity in vitro. These data provide evidence for a role for both autophosphorylation and phosphatase activities of FixL in the mechanism by which oxygen tension within the alfalfa nodule induces expression of bacterial nitrogen fixation genes during symbiosis.