Carotenoid binding to photosystems I and II of Chlamydomonas reinhardtii cells grown under weak light or exposed to intense light

Thylakoids, isolated from Chlamydomonas reinhardtii cells which were grown under weak light or exposed to a short period of intense light, were solubilised with decanoyl-N-methylglucamide (Mega 10) or with a new related detergent, dodecanoyl-N-hydroxyethylglucamide (DODHEGA) to prepare photosystem I and II complexes or sub-complexes for pigment stoichiometry determination. Loroxanthin was only bound to photosystem II antenna but most of the lutein, the other main alpha -carotene derived xanthophyll, was also associated with photosystem II antenna, as well as with the photosystem I antenna in which it represented the main xanthophyll species. The mean xanthophyll composition of the photosystem II antenna, prepared both by partially denaturing electrophoresis or sucrose gradient sedimentation, showed that more than three molecules (most probably four) were associated with individual light-harvesting complex II (LHCII) polypeptide (on the basis of seven chl a per polypeptide). Intense light exposition of the cells resulted in increased levels of lutein and of components of the violaxanthin cycle, part of them being only loosely bound to pigment protein complexes. The violaxanthin cycle components were recovered in PSI and PSII whereas the additional level of lutein was restricted to PSII sub-complexes. (C) 2001 Editions scientifiques et medicales Elsevier SAS.