Unexpected NO-dependent DNA binding by the CooA homolog from Carboxydothermus hydrogenoformans

CooA, the CO-sensing heme protein from Rhodospirillum rubrum, regulates the expression of genes that encode a CO-oxidation system, allowing R. rubrum to use CO as a sole energy source. To better understand the gas-sensing regulation mechanism used by R. rubrum CooA and its homologs in other organisms, we characterized spectroscopically and functionally the Fe(II), Fe(II)-NO, and Fe(II)-CO forms of CooA from Carboxydothermus hydrogenoformans. Surprisingly, and unlike R. rubrum CooA, C. hydrogenoformans CooA binds NO to form a six-coordinate Fe(II)-NO heme that is active for DNA binding in vitro and in vivo. In contrast, R. rubrum CooA, which is exquisitely specific for CO, forms a five-coordinate Fe(II)-NO adduct that is inactive for DNA binding. Based on analyses of protein variants and temperature studies, NO-dependent DNA binding by C. hydrogenoformans CooA is proposed to result from a greater apparent stability of the six-coordinate Fe(II)-NO adduct at room temperature. Results from the present study strengthen the proposal that CO specificity in the CooA activation mechanism is based on the requirement for a small, neutral distal ligand, which in turn affects the relative positioning of the ligand-bound heme.