Rapid electron transfer to photosystem I and unusual spectral features of cytochrome c(6) in Synechococcus sp PCC 7002 in vivo

Cytochrome c(6) donates electrons to photosystem I (PS I) in Synechococcus sp. PCC 7002. In this work., we provide evidence for rapid electron transfe r (t(1/2) = 3 mus) from cytochrome c(6) to PS I in this cyanobacterium in v ivo, indicating prefixation of the reduced donor protein to the photosystem . We have investigated the cytochrome c(6)-PS I interaction by laser flash- induced spectroscopy of intact and broken cells and by redox titrations of membrane and supernatant fractions. Redox studies revealed the expected mem brane-bound cytochrome f, b(6), and b(559) species and two soluble cytochro mes with alpha -band absorption peaks of 551 and 553 nm and midpoint potent ials of - 100 and 370 mV. respectively. The characteristics and the symmetr ical alpha -band spectrum of the latter correspond to typical cyanobacteria l cytochrome c(6) proteins. Rapid oxidation of cytochrome c(6) by PS I in v ivo results in a unique, asymmetric oxidation spectrum, which differs signi ficantly from the spectra obtained for cytochrome c(6) in solution. The bas is for the unusual cytochrome c(6) spectrum and possible mechanisms of cyto chrome c(6) fixation to PS I are discussed. The occurrence of rapid electro n transfer to PS I in cyanobacteria suggests that this mechanism evolved be fore the endosymbiotic origin of chloroplasts. Its selective advantage may lie in protection against photo-oxidative damage as shown for Chlamydomonas .