The objective of this work is the identification of additional regulatory elements required for optimal microaerobic expression of R. etli CFN42 fixKf and fixNd genes. The experimental strategies for the identification of these elements included random mutagenesis and complementation experiments. The results showed that R. etli CFN42 FixL is unable to activate fixKf expression directly. Different derivatives of R. etli CFN42 cured of different plasmids or with deletions on pSym were analyzed. The results obtained suggest the existence of different genetic elements required for an optimal microaerobic fixKf expression, and that should be encoded on pSym and pCFN42f. On the other hand, the expression of fixNOQPd is under the control of a complex regulatory system, involving the participation of FixL, FixKf and FnrN (López et al. submitted); by a mutagenesis strategy we isolated an interesting mutant that affects drastically fixNd expression. However, this mutation does not affect fixKf expression, suggesting that is affecting a novel element, acting downstream of fixKf in the cascade for fixNd expression.
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