Regulation of accumulation of the major thylakoid polypeptides in Chlamydomonas reinhardtii y-1 at 25 degrees C and 38 degrees C.

Abstract

The amount of mRNA for polypeptides of the chlorophyll a/b-protein complex of thylakoid membranes in etiolated and greening cells of C. reinhardtii y-1 was examined by immunoprecipitation and electrophoresis of products of in vitro translation to determine at which stage production of these polypeptides is regulated. Cells grown 4 days in the dark at 25.degree. C contained small amounts of translatable mRNA for the major membrane polypeptides. Exposure of these etiolated cells to light, under conditions in which the membrane polypeptides accumulated, resulted in a significant increase in the quantity of the mRNA. In contrast, when etiolated cells were incubated for 1-2 h in the dark at 38.degree. C, translation assays indicated that mRNA for the membrane polypeptides became abundant. Moreover, the quantity of the mRNA did not increase when these cells subsequently were exposed to light. Therefore, at 38.degree. C the cellular level of the polypeptides is not regulated by synthesis of mRNA. The in vitro synthesized polypeptides, which were precipitated with antibodies prepared against the purified thylakoid polypeptides, had apparent MW of 31,500 and 30,000. The corresponding immunoprecipitated polypeptides made in vivo had apparent MW of 29,500 and 26,000. Thus, the membrane polypeptides are made as precursors. No net accumulation of the polypeptides occurred in cells in the dark at 38.degree. C, but immunoreactive polypeptides the size of the mature membrane components were labeled during incubation of cells with [14C]acetate in the dark. The mRNA was translated in the dark, but since the polypeptides did not accumulate, the products of translation were probably degraded. At 25.degree. C production of the polypeptides is regulated by the level of translatable mRNA in the cells. At 38.degree. C, however, the accumulation of the polypeptides is controlled by posttranslational processes.