Rapid Electron Transfer to Photosystem I and Unusual Spectral Features of Cytochromec6inSynechococcussp. PCC 7002 in Vivo

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Abstract
Cytochrome c6 donates electrons to photosystem I (PS I) in Synechococcus sp. PCC 7002. In this work, we provide evidence for rapid electron transfer (t1/2 = 3 μs) from cytochrome c6 to PS I in this cyanobacterium in vivo, indicating prefixation of the reduced donor protein to the photosystem. We have investigated the cytochrome c6−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, b6, and b559 species and two soluble cytochromes with α-band absorption peaks of 551 and 553 nm and midpoint potentials of −100 and 370 mV, respectively. The characteristics and the symmetrical α-band spectrum of the latter correspond to typical cyanobacterial cytochrome c6 proteins. Rapid oxidation of cytochrome c6 by PS I in vivo results in a unique, asymmetric oxidation spectrum, which differs significantly from the spectra obtained for cytochrome c6 in solution. The basis for the unusual cytochrome c6 spectrum and possible mechanisms of cytochrome c6 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.