Here we describe and discuss this new approach and present one example in which the system can be used to follow the decline of the proteins of PSII and PSI and their associated light-harvesting complexes when chloroplast protein synthesis ceases. Although, protein turnover is normally studied through pulse and pulse-chase experiments with labeled methionine [13] or by using antibiotics which specifically inhibit translation [14], these methods are not suitable for the study of the light-harvesting complexes in Chlamydomonas for several reasons. First, protein labeling of Chlamydomonas with amino acids is only possible with labeled arginine but not with methionine although cells can be labeled with S35-sulfate or C14-acetate [1]. Second the very low turnover
of the light harvesting systems makes the interpretation of pulse labeling and pulse-chase experiments difficult. Third, secondary effects of antibiotics cannot be excluded especially if they are used for long periods. Because repression of chloroplast gene expression occurs gradually in this system, it allows one to extend the time scale which
is particularly useful when processes that occur sequentially in time are studied.We have taken advantage of our newly developed repressible chloroplast gene expression system for monitoring the fate of the core PSII and PSI subunits and of individual components of their lightharvesting complexes upon inhibition of chloroplast protein synthesis.