Donor‐side photoinhibition in photosystem II from Chlamydomonas reinhardtiiupon mutation of tyrosine‐Z in the D1 polypeptide to phenylalanine

When tyrosine‐Z of the D1‐polypeptide of the photosystem II from Chlamydomonas reinhardtiiwas changed to phenylalanine, the rapid donor to P680+was lost, and P680+accumulated on illumination. The rapid donation from tyrosine‐Z was replaced by a slow electron transfer from an endogenous donor. Spectrophotometric measurements showed that carotenoids and chlorophylls were bleached by the P680+either directly or indirectly upon illumination. The carotenoid bleaching was inhibited in the presence of SOD or catalase, but the reaction did not require molecular oxygen as an electron acceptor. These observations led us to conclude that active oxygen radicals, possibly hydroxyl radicals, take part in the destruction of carotenoids in the Y161F mutant. Possible mechanisms for the destruction are discussed.