Rapid Electron Transfer to Photosystem I and Unusual Spectral Features of Cytochrome c sub(6) in Synechococcus sp. PCC 7002 in Vivo

• Published in: Biochemistry (Easton) Vol. 40; no. 35; pp. 10570 - 10577
• Main Authors: Baymann, F, Rappaport, F, Joliot, P, Kallas, T
• Format: Journal Article
• Language: English
• Published: 01-09-2001
• Subjects: Cyanophyta; cytochrome c6; Synechococcus
• ISSN: 0006-2960
• DOI: 10.1021/bi010194a

Cytochrome c sub(6) donates electrons to photosystem I (PS I) in Synechococcus sp. PCC 7002. In this work, we provide evidence for rapid electron transfer (t sub( one half ) = 3 mu s) from cytochrome c sub(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 sub(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 sub(6), and b sub(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 sub(6) proteins. Rapid oxidation of cytochrome c sub(6) by PS I in vivo results in a unique, asymmetric oxidation spectrum, which differs significantly from the spectra obtained for cytochrome c sub(6) in solution. The basis for the unusual cytochrome c sub(6) spectrum and possible mechanisms of cytochrome c sub(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.