The photosynthetic electron transport chain of a mutant strain of Chlamydomonas reinhardi lacking P700 activity

Abstract

Components and reactions of the photosynthetic electron transport chain were investigated in a mutant strain of the unicellular green alga Chlamydomonas reinhardi which is virtually devoid of the System I reaction center pigment, P700. The plastocyanin and ferredoxin isolated from this mutant strain are both qualitatively and quantitatively indistinguishable from that isolated from the wild-type strain. Cytochromes with absorption maxima at 553 and 559 nm cannot be oxidized by far-red light in the mutant strain, but they are reduced by red light. The Fe(CN) 6 3−-Hill reaction in the mutant strain is about 50% of that of wild type at high light intensities; however, at low light levels, it is not significantly different from the rate of wild type. These results are interpreted to indicate that P700 is not so closely involved or complexed with adjacent electron carriers or with the reaction center of System II that destruction of P700 necessarily leads to alteration of these other components of the electron transport chain. It is suggested that the Hill reaction data can be explained by the existence of two separate sites for photoreduction of Fe(CN) 6 3− in wild type, whereas only one remains operative in the mutant strain.