Reactions of isocytochrome c(2) in the photosynthetic electron transfer chain of Rhodobacter sphaeroides

Rhodobacter sphaeroides strains lacking cytochrome c(2) (cyt c(2)), the normal electron donor to P-870(+) in light-oxidized reaction center (RC) complexes, are unable to grow photosynthetically. However, spd mutations that suppress the photosynthetic deficiency of cyt ct mutants elevate levels of the cyt c(2) isoform, isocyt c(2). We monitored photosynthetic electron transfer in whole cells, in chromatophores, and with purified components to ascertain if and how isocyt c(2) reduced light-oxidized RC complexes. These studies revealed that several fundamental aspects of photosynthetic electron transfer were similar in strains that use isocyt c(2) and wild-type cells. For example, P-870(+) reduction accompanied cytochrome c oxidation. In addition, photosynthetic electron transfer was blocked by the well-known cyt bc(1) complex inhibitors antimycin and myxothiazol. However, even at the increased isocyt c(2) levels present in these strains (similar to 40% that of cyt c(2) in wild-type cells), there was little, if any, of the rapid (<5 mu s) electron transfer to P-870(+) that is characteristic of cytochromes bound to RC complexes at the time of the light flash. Thus, it appears that isocyt c(2) function limits the in vivo rate of P-870(+) reduction. Indeed, at low ionic strength in vitro, the apparent affinity of isocyt c(2) for RC complexes (K-D similar to 40 mu M) is significantly lower than that of cyt c(2) (K-D similar to 1.0 mu M). This reduced affinity does not appear to result from an altered mode of RC binding by isocyt c(2) since electrostatic interactions make similar overall contributions to the binding of both cyt c(2) and isocyt c(2) to this membrane-bound redox partner. Thus, sequence, structural, or local conformational differences between cyt c(2) and isocyt c(2) significantly alter their apparent affinities for this physiologically relevant redox partner.