Cyanobacterial cytochrome c(M): probing its role as electron donor for Cu(A) 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 c(6) (Cytc(6)) or plastocyanin (PC). By contrast, the interaction of an additional redox carrier, cytochrome c(M) (Cytc(M)), with either PET or RET is still under discussion. Here, we focus on the (putative) role of Cytc(M) in cyanobacterial respiration. It is demonstrated that genes encoding the main terminal oxidase (cytochrome c oxidase, COX) and cytochrome c(M) are found in all 44 totally or partially sequenced cyanobacteria (except one strain). In order to check whether Cytc(M) can act as electron donor to COX, we investigated the intermolecular electron transfer kinetics between Cytc(M) and the soluble Cu(A) 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)x10(5) M(-1) s(-1) and (9.6+/-0.4)x10(3) M(-1) s(-1) (5 mM phosphate buffer, pH 7, 50 mM KCl). A comparative analysis with Cytc(6) and PC demonstrates that Cytc(M) functions as electron donor to Cu(A) as efficiently as Cytc(6) but more efficient than PC. Furthermore, we demonstrate the association of Cytc(M) with the cytoplasmic and thylakoid membrane fractions by immunobloting and discuss the potential role of Cytc(M) as electron donor for COX under stress conditions.