Rapid electron transfer to photosystem I and unusual spectral features of cytochrome c6 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 transfer (t(1/2) = 3 mus) from cytochrome c(6) to PS I in this cyanobacterium in vivo, 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 membrane-bound cytochrome f, b(6), and b(559) species and two soluble cytochromes with alpha -band absorption peaks of 551 and 553 nm and midpoint potentials of - 100 and 370 mV. respectively. The characteristics and the symmetrical alpha -band spectrum of the latter correspond to typical cyanobacterial cytochrome c(6) proteins. Rapid oxidation of cytochrome c(6) by PS I in vivo results in a unique, asymmetric oxidation spectrum, which differs significantly from the spectra obtained for cytochrome c(6) in solution. The basis for the unusual cytochrome c(6) spectrum and possible mechanisms of cytochrome c(6) fixation to PS I are discussed. The occurrence of rapid electron transfer to PS I in cyanobacteria suggests that this mechanism evolved before the endosymbiotic origin of chloroplasts. Its selective advantage may lie in protection against photo-oxidative damage as shown for Chlamydomonas.