Xanthophyll cycle mutants from Chlamydomonas reinhardtii indicate a role for zeaxanthin in the D1 protein turnover

Photosynthetic activity, pigment conversion and D1 protein degradation unde r high light stress has been investigated in a wild type strain and two xan thophyll cycle mutants (npq1 and npq2) of Chlamydomonas reinhardtii. Wild t ype cells exhibited the well-known inactivation of photosystem II in high l ight, which was accompanied by the loss of B-carotene and a concomitant inc rease of zeaxanthin. Complete degradation of DI protein was found after 2 h of illumination in the presence of chloramphenicol, an inhibitor of chloro plast protein synthesis. The npq1 mutant, which is unable to convert violax anthin to zeaxanthin, showed a very similar behaviour. For the npq2 mutant, however, which is unable to form violaxanthin from zeaxanthin and thus con tains high amounts of zeaxanthin even in low light, photosystem II inactiva tion was less pronounced. This was paralleled by a much slower D1 protein d egradation in chloramphenicol treated cells. Our results support a protecti ve role for zeaxanthin against high light-induced photosystem II inactivati on resulting in a slowed-down D1 protein turnover. (C) 2000 Editions scient ifiques et medicales Elsevier SAS.