Nitrogen fixation in Rhizobia is regulated via control of transcription of fix and nif genes, under low oxygen conditions (Soupene et al. 1995). The regulatory elements identified so far (FixL, FixJ, FixK, FnrN, NifA and RpoN) participate in regulatory cascades whose architecture is species-, or even strain-specific. In one cascade, NifA and RpoN control the expression of genes coding for nitrogenase. Another cascade involves the two-component system formed by FixL-FixJ and FixK (Fischer 1994) and is specific for symbiotic diazotrophs. This cascade controls the expression of the fixNOQP operon, which codes for a terminal oxidase with a high affinity for oxygen, necessary for bacteroid survival in the nodule (Preisig et al. 1993). In Rhizobium etli CFN42, two copies of the fixN and fixK genes are present, one located on the pSym and the other on a cryptic plasmid (pCFN42f); this plasmid also harbors a fixL gene coding for an atypical FixL protein: sequence analysis of the predicted FixL polypeptide suggests that it is a probable hybrid histidine-kinase, two-component sensor protein. An interesting characteristic of this strain is the absence of a structurally similar fiixJ gene (Girard et al. 2000). This led us to hypothesize the existence of another protein, forming with FixL a two-component regulatory system.

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