In vitro studies on light-induced inhibition of Photosystem II and D1-protein degradation at low temperatures

Abstract

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.