COOA, A CO-SENSING TRANSCRIPTION FACTOR FROM RHODOSPIRILLUM-RUBRUM, IS A CO-BINDING HEME PROTEIN

Biological sensing of small molecules such as NO, O-2, and CO is an im portant area of research; however, little is know about how CO is sens ed biologically, The photosynthetic bacterium Rhodospirillum rubrum re sponds to CO by activating transcription of two operons that encode a CO-oxidizing system. A protein, CooA has been identified as necessary for this response, CooA is a member of a family of transcriptional reg ulators similar to the cAMP receptor protein and fumavate nitrate redu ction from Escherichia coli, In this study we report the purification of wild-type CooA from its native organism, R, rubrum, to greater than 95% purity, The purified protein is active in sequence-specific DNA b inding in the presence of CO, but not in the absence of CO, Gel filtra tion experiments reveal the protein to be a dimer in the absence of CO , Purified CooA contains 1.6 mol heme per mol of dimer, Upon interacti ng with CO, the electronic spectrum of CooA is perturbed, indicating t he direct binding of CO to the heme of CooA. A hypothesis for the mech anism of the protein's response to CO is proposed.