HEAT-SHOCK IN CHLAMYDOMONAS-REINHARDTII - BREAKDOWN OF D1-POLYPEPTIDES AND D2-POLYPEPTIDES OF PHOTOSYSTEM-II REACTION-CENTER

Algal cells of Chlamydomonas reinhardtii were incubated for 90 min at 41.5 degrees C (heat shock) in light (180 mu mol photons m(-2) s(-1)) or in darkness. D1-degradation products of 27 and 25 kDa accumulated i n thylakoid membranes during heat shock conditions under light as well as in darkness, whereas an intermediate of 15 kDa accumulated during heat shock under concomitant light. Two D2 membrane-bound breakdown pr oducts measuring 26 and 24 kDa accumulated during heat shock in darkne ss. From these results it is concluded that: (i) heat shock itself (ir respective of light or darkness) does not influence the activity of an endoprotease that, under normal growth conditions, degrades damaged D 1-molecules, resulting in vivo in thylakoid membrane-bound breakdown p roducts of 27 and 25 kDa; (ii) due to the inactivation of another prot ease these two intermediates accumulate during heat shock light as wel l as darkness; (iii) during heat shock in darkness an additional prote ase is inactivated, resulting in the accumulation of a 15 kDa intermed iate, and (iv) D1- and D2-polypeptides of the photosystem II reaction centre seem to be degraded by different proteolytic systems.