The turnover in vivo of the Photosystem II (PS II) reaction center D1 protein was investigated by [35S] methionine labeling of leaf discs of Brassica napus and subsequent analysis after thylakoid SDS-gel electrophoresis. The rate of D1 protein degradation was found to have a t1/2 of approximately 2 h, at an irradiance corresponding to the growth irradiance. The rate of D1 protein degradation was not increased further by prior photoinhibitory treatment which inactivated 40% of the PS II centers, but the amount of [35S]methionine label incorporated into the D1 protein during 1 h was increased 3-fold. Whether photoinhibited or not, the incorporation of label into the D1 protein increased with irradiance only up to approximately the growth irradiance and above that it decreased with increasing incident light. These data indicate that a high rate of D1 protein degradation occurs not only after photoinhibition but also under conditions where no net decrease in the number of functional PS II centers occurs and that the number of PS II centers that undergo D1 protein turnover varies depending on prior photoinhibitory damage and on incident irradiance. A down-regulation of PS II in leaves at high irradiances is discussed.
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