Fine-Tuning Modulation of Oxidation-Mediated Posttranslational Control of Bradyrhizobium diazoefficiens FixK 2 Transcription Factor.

FixK 2 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 . Aside from the balanced expression of the fixK 2 gene under microoxic conditions (induced by the two-component regulatory system FixLJ and negatively auto-repressed), FixK 2 activity is posttranslationally controlled by proteolysis, and by the oxidation of a singular cysteine residue (C183) near its DNA-binding domain. To simulate the permanent oxidation of FixK 2 , we replaced C183 for aspartic acid. Purified C183D FixK 2 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 FixK 2 , expression of a fixNOQP'-'lacZ fusion was similar to that in the wild type, when both strains were grown microoxically. The C183D FixK 2 encoding strain also showed a wild-type phenotype in symbiosis with soybeans, and increased fixK 2 gene expression levels and FixK 2 protein abundance in cells. These two latter observations, together with the global transcriptional profile of the microoxically cultured C183D FixK 2 encoding strain, suggest the existence of a finely tuned regulatory strategy to counterbalance the oxidation-mediated inactivation of FixK 2 in vivo.