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Gene Review

fixL  -  FixL oxygen regulated histidine kinase

Sinorhizobium meliloti 1021

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Disease relevance of fixL

  • We report the discovery of two genes from Rhizobium meliloti, fixL and fixJ, which are positive regulators of symbiotic expression of diverse nitrogen fixation (nif and fix) genes. nif gene regulation is shown to consist of a cascade: the fixLJ genes activate nifA, which in turn activates nifHDK and fixABCX [1].
  • In this work, deletion derivatives of fixL* were constructed and overexpressed in Escherichia coli, and the truncated proteins were purified [2].
  • A gene bank of Azorhizobium caulinodans DNA constructed in the bacteriophage lambda GEM11 was screened with Rhizobium meliloti fixL and fixJ genes as probes [3].
  • The regulatory status of the fixL- and fixJ-like genes in Bradyrhizobium japonicum may be different from that in Rhizobium meliloti [4].

High impact information on fixL


Biological context of fixL

  • Individual R. leguminosarum bv. viciae fixK and fixL insertion mutants displayed a Fix+ phenotype [6].
  • The R. leguminosarum bv. viciae nifA gene is expressed independently of FixL and FixK under aerobic and microaerobic conditions, whereas fixL gene expression is induced under microaerobiosis [6].
  • Kanamycin cartridges were inserted into the cloned fixL and fixJ genes and recombined into the host genome [3].
  • The genes responsible for sensing and transducing the low oxygen signal, fixL and fixJ, encode proteins (FixL and FixJ, respectively) that are homologous to a large family of bacterial proteins involved in signal transduction, the two component regulatory system proteins [7].

Regulatory relationships of fixL

  • 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 [8].

Other interactions of fixL

  • The remaining seven loci showed fixL- and fixJ-independent induction of expression in response to oxygen limitation [9].
  • Another orf was identified down-stream of fixK-fixL and encodes a product which has homology to pseudoazurins from different species [6].


  1. Cascade regulation of nif gene expression in Rhizobium meliloti. David, M., Daveran, M.L., Batut, J., Dedieu, A., Domergue, O., Ghai, J., Hertig, C., Boistard, P., Kahn, D. Cell (1988) [Pubmed]
  2. The FixL protein of Rhizobium meliloti can be separated into a heme-binding oxygen-sensing domain and a functional C-terminal kinase domain. Monson, E.K., Weinstein, M., Ditta, G.S., Helinski, D.R. Proc. Natl. Acad. Sci. U.S.A. (1992) [Pubmed]
  3. Involvement of fixLJ in the regulation of nitrogen fixation in Azorhizobium caulinodans. Kaminski, P.A., Elmerich, C. Mol. Microbiol. (1991) [Pubmed]
  4. The regulatory status of the fixL- and fixJ-like genes in Bradyrhizobium japonicum may be different from that in Rhizobium meliloti. Anthamatten, D., Hennecke, H. Mol. Gen. Genet. (1991) [Pubmed]
  5. A haemoprotein with kinase activity encoded by the oxygen sensor of Rhizobium meliloti. Gilles-Gonzalez, M.A., Ditta, G.S., Helinski, D.R. Nature (1991) [Pubmed]
  6. Rhizobium leguminosarum bv. viciae contains a second fnr/fixK-like gene and an unusual fixL homologue. Patschkowski, T., Schlüter, A., Priefer, U.B. Mol. Microbiol. (1996) [Pubmed]
  7. Genetic regulation of nitrogen fixation in Rhizobium meliloti. Cebolla, A., Palomares, A.J. Microbiologia (1994) [Pubmed]
  8. Autophosphorylation and phosphatase activities of the oxygen-sensing protein FixL of Rhizobium meliloti are coordinately regulated by oxygen. Lois, A.F., Weinstein, M., Ditta, G.S., Helinski, D.R. J. Biol. Chem. (1993) [Pubmed]
  9. Isolation and regulation of Sinorhizobium meliloti 1021 loci induced by oxygen limitation. Trzebiatowski, J.R., Ragatz, D.M., de Bruijn, F.J. Appl. Environ. Microbiol. (2001) [Pubmed]
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