LIGHT-INDUCED DEGRADATION OF PHOTOSYSTEM-2 REACTION-CENTER PROTEINS D1 AND D2

The phenomenon of photoinhibition, by which all organisms performing o xygenic photosynthesis decrease their photosynthetic activity when exp osed to excessive irradiance, depends, at molecular level, mainly on l ight-induced modifications of photosystem 2 (PS2). In the last decade a great deal of research has been focused on the molecular mechanisms responsible for these modifications and some of the processes involved have been clarified. Inactivation of PS2 and degradation of its compo nents, as well as some of the mechanisms involved in the restoration o f PS2 activity can be studied in vitro using preparations characterize d by various degree of complexity from entire thylakoid membrane to th e simplest preparation of isolated PS2 reaction centre. Although the i n vitro approach contributes essential information to our understandin g of the molecular process underlying photoinhibition, extrapolation t o the in vivo situation is not obvious and many points demand further experimental work.