Beta-carotene to zeaxanthin conversion in the rapid turnover of the D1 protein of photosystem II.

The carotenoid composition was investigated during enhanced D1 protein turnover in Chlamydomonas reinhardtii exposed to high light. After 2 h of high light there was no loss of the D1 protein yet. However, the beta-carotene content was significantly reduced. In parallel, an increase of the zeaxanthin content was found, which was higher than can be accounted for by the light-induced de-epoxidation of violaxanthin in the xanthophyll cycle reactions. We therefore assume that beta-carotene of photosystem II (PS II) is hydroxylated to zeaxanthin under high light stress. Inhibitors of carotene biosynthesis led to the loss of both PS II activity and D1 protein, indicating the requirement of beta-carotene synthesis for the reassembly of PS II in high light. Diuron blocked D1 protein as well as beta-carotene turnover. In the presence of chloramphenicol -- which allows just one turnover of the D1 protein -- 15% of the total beta-carotene was lost, calculated to be two beta-carotene.