Modular structure of the FixL protein of Rhizobium meliloti.

FixL protein of Rhizobium meliloti is a haemo-protein kinase which activates the transcription of nifA and fixK genes via the transcriptional activator protein FixJ under microaerobic conditions. FixL and FixJ proteins belong to the family of two-component regulatory systems for which primary sequence data predicts a modular structure. We showed, using Escherichia coli as heterologous host, that FixL indeed has a modular structure. The amino-terminal hydrophobic domain is dispensable for the oxygen-regulated activity of FixL in vivo. The central cytoplasmic non-conserved domain is necessary for the oxygen-sensing function of FixL whereas it is not necessary for the activation of FixJ by FixL. We propose that, under aerobic conditions, the central domain represses the activating function associated with the carboxy-terminal conserved domain.