Synthesis and turnover of photosystem II reaction center protein D1. Ribosome pausing increases during chloroplast development.

The chloroplast photosystem II reaction center protein D1 contains five membrane-spanning helices and binds chlorophyll, carotenoid, quinone, iron, and probably manganese. Turnover of pulse-labeled D1 in isolated plastids was found to involve cleavage between helix IV and helix V, which releases a 23-kDa N-terminal peptide and two C-terminal peptides of 10 and 8 kDa. Ribosomes pause at specific sites during translation of D1, which results in the accumulation of D1 translation intermediates. Pulse-labeling assays followed by polysome isolation and immunoprecipitation identified paused D1 translation intermediates of 9, 12.5, 15-18, 20, 21, 24, and 28-32 kDa. Ribosome pausing was not altered when dark-grown seedlings were illuminated for up to 1 h, even though this treatment stimulated accumulation of chlorophyll and D1. However, illumination of plants for 16-72 h resulted in increased ribosome pausing and the build-up of D1 translation intermediates. We hypothesize that ribosome pausing during synthesis of D1 improves the efficiency of chlorophyll binding of D1 nascent chains and enhances accumulation of D1 in mature chloroplasts, which have reduced rates of chlorophyll biosynthesis.