Cyanobacterial cytochrome cM: Probing its role as electron donor for CuA of cytochrome c oxidase

It is well known that efficient functioning of photosynthetic (PET) and respiratory electron transport (RET) in cyanobacteria requires the presence of either cytochrome c6 (Cytc6) or plastocyanin (PC). By contrast, the interaction of an additional redox carrier, cytochrome cM (CytcM), with either PET or RET is still under discussion. Here, we focus on the (putative) role of CytcM in cyanobacterial respiration. It is demonstrated that genes encoding the main terminal oxidase (cytochrome c oxidase, COX) and cytochrome cM are found in all 44 totally or partially sequenced cyanobacteria (except one strain). In order to check whether CytcM can act as electron donor to COX, we investigated the intermolecular electron transfer kinetics between CytcM and the soluble CuA domain (i.e. the donor binding and electron entry site) of subunit II of COX. Both proteins from Synechocystis PCC6803 were expressed heterologously in E. coli. The forward and the reverse electron transfer reactions were studied yielding apparent bimolecular rate constants of (2.4 ± 0.1) × 105 M− 1 s− 1 and (9.6 ± 0.4) × 103 M− 1 s− 1 (5 mM phosphate buffer, pH 7, 50 mM KCl). A comparative analysis with Cytc6 and PC demonstrates that CytcM functions as electron donor to CuA as efficiently as Cytc6 but more efficient than PC. Furthermore, we demonstrate the association of CytcM with the cytoplasmic and thylakoid membrane fractions by immunobloting and discuss the potential role of CytcM as electron donor for COX under stress conditions.