Fine‐tuning modulation of oxidation-mediated posttranslational control of Bradyrhizobium diazoefficiens FixK₂ transcription factor

FixK₂ is a CRP/FNR‐type transcription factor that plays a central role in a sophisticated regulatory network for the anoxic, microoxic and symbiotic lifestyles of the soybean endosymbiont Bradyrhizobium diazoefficiens. Apart of the balanced expression of the fixK₂ gene under microoxic conditions (induced by the two‐component regulatory system FixLJ and negatively auto‐repressed), FixK₂ activity is posttranslationally controlled by proteolysis, and by oxidation of a singular cysteine residue (C183) near its DNA‐binding domain. To simulate permanent oxidation of FixK₂, we replaced C183 for aspartic acid. Purified C183D FixK₂ protein showed both low DNA binding and in vitro transcriptional activation from the promoter of the fixNOQP operon, required for respiration under symbiosis. However, in a B. diazoefficiens strain coding for C183D FixK₂, expression of a fixNOQP’‐‘lacZ fusion was similar to that in the wild type, when both strains were grown microoxically. The C183D FixK₂ encoding strain also showed a wild‐type phenotype in symbiosis with soybeans, and increased fixK₂ gene expression levels and FixK₂ protein abundance in cells. These two latter observations together with a global transcriptional profile of the microoxically cultured C183D FixK₂ encoding strain suggest the existence of a finely tuned regulatory strategy to counterbalance the oxidation‐mediated inactivation of FixK₂ in vivo.