INVITRO STUDIES ON LIGHT-INDUCED INHIBITION OF PHOTOSYSTEM-II AND D1-PROTEIN DEGRADATION AT LOW-TEMPERATURES

In order to get information on the molecular background behind the aggrevated photodamage to photosynthesis at low temperatures and to investigate the general mechanism of D1-protein degradation, isolated spinach thylakoids were subjected to photoinhibitory treatment at various temperatures. The results reveal that: (i) the Photosystem II electron transport per se is less sensitive to high light at low temperatures in contrast to the overall photosynthetic process; (ii) the degradation of D1-protein is severely retarded below 7°C; (iii) inhibition of Photosystem II electron transport and D1-protein degradation are separate events the two reactions could be completely separated in time; (iv) D1-protein is degraded by enzymatic proteolysis and not by a direct photocleavage reaction; (v) degradation of the D1-protein readily proceeds in the dark but its triggering for the proteolytic attack requires light; (vi) strong illumination at low temperature does not induce any lateral rearrangement in the location of Photosystem II; and (vii) D1-protein fragments can be identified in vitro and be used to verify the specificity of D1-protein degradation under various experimental conditions. © 1990.