Nature 350, 170 - 172 (14 March 1991); doi:10.1038/350170a0

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

Marie A. Gilles-Gonzalez, Gary S. Ditta & Donald R. Helinski

Department of Biology and Center for Molecular Genetics, 0634, University of California, San Diego, La Jolla, California 92093, USA

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 -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.

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