Crystal Structure of CO-sensing Transcription Activator CooA Bound to Exogenous Ligand Imidazole

CooA is a CO-dependent transcriptional activator and transmits a CO-sensing signal to a DNA promoter that controls the expression of the genes responsible for CO metabolism. CooA contains a b-type heme as the active site for sensing CO. CO binding to the heme induces a conformational change that switches CooA from an inactive to an active DNA-binding form. Here, we report the crystal structure of an imidazole-bound form of CooA from Carboxydothermus hydrogenoformans (Ch-CooA). In the resting form, Ch-CooA has a six-coordinate ferrous heme with two endogenous axial ligands, the α-amino group of the N-terminal amino acid and a histidine residue. The N-terminal amino group of CooA that is coordinated to the heme iron is replaced by CO. This substitution presumably triggers a structural change leading to the active form. The crystal structure of Ch-CooA reveals that imidazole binds to the heme, which replaces the N terminus, as does CO. The dissociated N terminus is positioned approximately 16 Å from the heme iron in the imidazole-bound form. In addition, the heme plane is rotated by 30° about the normal of the porphyrin ring compared to that found in the inactive form of Rhodospirillum rubrum CooA. Even though the ligand exchange, imidazole-bound Ch-CooA remains in the inactive form for DNA binding. These results indicate that the release of the N terminus resulting from imidazole binding is not sufficient to activate CooA. The structure provides new insights into the structural changes required to achieve activation.