Characterization of the photosynthetic electron transport chain in normal and photobleached Anabaena cylindrica by flash spectroscopy.

Electron transport of normal and photobleached Anabaena cylindrica was studied using spectral and kinetic analyses of absorbance transients induced by single turnover flashes. Between 500 and 600 nm two positive bands (approximately 540 and approximately 566 nm) and two negative bands (approximately 515 and approximately 554 nm) were found. Absorbance changes at 515 and 540 nm were partly characterized. None of these absorbance changes represent an electrochromic shift. Absorbance changes at 554 and 566 nm correspond to the oxidation of cytochrome f and the reduction of cytochrome b563, respectively. We found a very slight 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) sensitivity of cytochrome f in normal cells, while DCMU was completely ineffective for cytochrome f reduction in photobleached cells. The absorbance change of cytochrome b563 increased, while the absorbance change of cytochrome f was smaller than in normal cells. The increased O2 evolution in photobleached cells and the negligible electron transport via cytochrome f suggest the participation of other electron acceptor(s) in the electron-transport chain of photobleached Anabaena cylindrica.