CO sensing and regulation of gene expression by the transcriptional activator CooA

The transcriptional activator CooA from Rhodospirillum rubrum contains a six-coordinate protoheme that acts as a CO sensor in vivo. CO is a physiological effector of CooA and replaces one of the axial ligands of the ferrous heme to form the GO-bound CooA that is active as the transcriptional activator. Cys(75) or His(77) is coordinated to the ferric and ferrous hemes in CooA, respectively. The redox-controlled ligand exchange between Cys(75) and His(77) proceeds during the change in the redox state of the heme. The reduction and oxidation midpoint potentials of CooA have been determined to be -320 and -260 mV, respectively. The properties of a functional chimera derived from CRP and CooA suggest that CooA activates the transcription by a similar mechanism to that for CRP at Class II CRP-dependent promoters. Alanine-scanning mutagenesis has revealed that Arg(24) and Arg(53) of CooA, which will be concerned with the protein-protein interaction with RNA polymerase, are critical amino acid residues for the transcriptional activator activity of CooA, and that Lys(26) and Asp(94) modulate the activity of CooA. (C) 2000 Elsevier Science BN. All rights reserved.