LIGHT-INDUCED TURNOVER OF THYLAKOID POLYPEPTIDES IN CHLAMYDOMONAS REINHARDI

Abstract

The turnover rate of the 32–34 kD and the light-harvesting complex (LHC) polypeptides of the photosynthetic membranes has been measured by radioactive pulse labeling and chase experiments in Chlamydomonas reinhardi cell cultures during logarithmic growth in the light. Loss of radioactivity from the various polypeptides was similar when 35 SO 4 or 14 C-acetate were used as radioactive tracers. However, the rate of 35 SO 4 incorporation into the 32–34 kD polypeptides, as compared to that of the LHC polypeptides, was higher than that of 14 C-acetate. The half-life of the LHC polypeptides was similar to the generation time and it can therefore be concluded that these polypeptides do not turn over significantly in dividing cells. The half-life of the 32–34 kD polypeptides was 3.5 h, or about one-third to one-quarter of the generation time. The radioactive labeling of the 32–34 kD polypeptides was considerably faster than expected from the rate of decay, and a plateau was reached after about two hours, while labeling of the LHC polypeptides continued. No loss of radioactivity from the 32–34 kD polypeptides could be detected during the first 3–4 h of chase immediately after pulse labeling. These results might suggest that the newly synthesized, membrane-bound 32–34 kD polypeptides appear first in the form of a small “transient pool” and are not immediately integrated in active photosystem II units where the light-induced turnover occurs. Turnover of the 32–34 kD polypeptides is markedly slower in dark-growing cells and is partially inhibited when cytoplasmic protein synthesis is inhibited. Inhibition of chloroplast protein synthesis during cell growth does not significantly affect the turnover of the 32–34 kD polypeptides.