Studies on Chloroplast Protein Synthesis In Vivo and In Vitro

Protein synthesis in the Chlamydomonas reinhardtii chloroplast was investigated by in vivo and in vitro approaches. The site of synthesis of chloroplast proteins was examined to determine how many proteins were synthesized inside the chloroplast. Cells were labeled in vivo with [35S]-sulfate in the presence of either. chloroplast or cytoplasmic protein synthesis inhibitors, and fractionated into soluble proteins, thylakoid membranes, and chloroplast ribosomal subunits. The use of high specific activity sulfate and improved gel electrophoretic techniques were key features of this study because they permitted minor chloroplast products to be detected for the first time. It was determined that twenty-eight to thirty-one polypeptides are synthesized in the chloroplast. These include the major thylakoid membrane proteins previously reported to be chloroplast products by Chua and Gillham (1977), but also include a number of minor and low-molecular weight thylakoid membrane proteins, three soluble proteins, and at least four chloroplast ribosomal proteins. The synthesis of chloroplast envelope constituents was not examined