Light regulates the rate of translation elongation of chloroplast reaction center protein D1

Abstract

Intact and lysed chloroplasts isolated from the day or night phase of seedling growth exhibit a higher rate of [35S]Met incorporation into the D1 protein m the light than in darkness. In the presence of the translation initiation inhibitor lincomycin, radiolabel incorporation remains unaffected for 7.5-10 min of the in vitro translation reaction, indicating that radiolabel incorporation is regulated by translation elongation. The rate of [35S]Met incorporation into D1-protein can be increased by addition of exogenous ATP to the in vitro translation reactions, however, ATP cannot replace light, and at physiological concentrations of stromal ATP (40 μM), the rate is at least 25-fold higher m the light than in darkness. This indicates that translation elongation is arrested in darkness. Separation of translation-elongation reactions into polysome-bound and membrane-integrated D1 proteins demonstrates that the rate of translation elongation is higher in the presence of light. In the light, less time is required to transiently radiolabel a D1 translation intermediate of about 17 kDa and to chase the translation intermediate into mature D1 protein. We propose that light regulates the enzymatic activity of the translation-elongation process in chloroplasts.