Soluble variants of Rhodobacter capsulatus membrane-anchored cytochrome cy are efficient photosynthetic electron carriers

Photosynthetic (Ps) electron transport pathways often contain multiple electron carriers with overlapping functions. Here we focus on two c-type cytochromes (cyt) in facultative phototrophic bacteria of the Rhodobacter genus: the diffusible cyt c2 and the membrane-anchored cyt c(y). In species like R. capsulatus, cyt c(y) functions in both Ps and respiratory electron transport chains, whereas in other species like R. sphaeroides, it does so only in respiration. The molecular bases of this difference was investigated by producing a soluble variant of cyt c(y) (S-c(y)), by fusing genetically the cyt c2 signal sequence to the cyt c domain of cyt c(y). This novel electron carrier was unable to support the Ps growth of R. capsulatus. However, strains harboring cyt S-c(y) regained Ps growth ability by acquiring mutations in its cyt c domain. They produced cyt S-c(y) variants at amounts comparable with that of cyt c2, and conferred Ps growth. Chemical titration indicated that the redox midpoint potential of cyt S-c(y) was about 340 mV, similar to that of cyts c2 or c(y). Remarkably, electron transfer kinetics from the cyt bc1 complex to the photochemical reaction center (RC) mediated by cyt S-c(y) was distinct from those seen with the cyt c2 or cyt c(y). The kinetics exhibited a pronounced slow phase, suggesting that cyt S-c(y) interacted with the RC less tightly than cyt c2. Comparison of structural models of cyts c2 and S-c(y) revealed that several of the amino acid residues implicated in long-range electrostatic interactions promoting binding of cyt c2 to the RC are not conserved in cyt c(y), whereas those supporting short-range hydrophobic interactions are conserved. These findings indicated that attaching electron carrier cytochromes to the membrane allowed them to weaken their interactions with their partners so that they could accommodate more rapid multiple turnovers.