Transcription factors FixK and FnrN are members of Crp family. In nitrogen-fixing Alphaproteobacteria, these homologous regulators activate expression of genes for adaptation to microaerobic conditions. Activation of genes is required for utilization of molecular oxygen and protection of oxygen-sensitive nitrogenase. Binding motifs for FnrN and FixK are undistinguisible. The main difference between FixK and FnrN is that FnrN can directly sence the presence of molecular oxygen by iron-sulfur cluster. FixK lacks such iron-sulfur cluster (and thus it is always on) however the fixK expression is activated by the oxygen-sensitive regulatory system FixL-FixJ .
We used comparative genomics approach to reconstruct transcriptional regulons controlled by FixK/FnrN regulators in Alphaproteobacteria.