A haemoprotein with kinase activity encoded by the oxygen sensor of Rhizobium meliloti

THE expression of the nitrogen-fixation genes of Rhizobium meliloti is controlled by oxygen^1,2. These genes are induced when the free oxygen concentration is reduced to microaerobic levels. Two regulator proteins, FixL and FixJ, initiate the oxygen-response cascade, and the genes that encode them have been cloned^2,3. The fixL product seems to be a transmembrane sensor that modulates the activity of the fixJ product, a cytoplasmic regulator³. FixL and FixJ are homologous to a family of bacterial two-component regulators⁴, for which the mode of signal transduc-tion is phosphorylation (reviewed in refs 5-9). We report here the purification of both FixJ and a soluble truncated FixL (FixL*), overproduced from a single plasmid construct. FixL* catalyses its own phosphorylation and the transfer of the γ-phosphate of ATP to Fix J. The resulting FixJ-phosphate linkage is sensitive to base, as are the aspartyl phosphates of homologous systems. Visible spectra of purified FixL* show that it is an oxygen-binding haemoprotein. We propose that FixL senses oxygen through its haem moiety and transduces this signal by controlling the phos-phorylation of FixJ.