Synthesis of a chloroplast membrane polypeptide on thylakoid-bound ribosomes during the cell cycle of Chlamydomonas reinhardii 137+

An in vitro protein synthesizing system consisting of thylakoid-bound polysomes supplemented with an S-100 from Escherichia coli has been used to investigate chloroplast membrane protein synthesis in synchronous cultures of Chlamydomonas reinhardii. The cell-free system produces at least one thylakoid membrane protein, D-2, on the basis of electrophoretic mobility and peptide mapping. Rough thylakoid membranes isolated in the middle and late light portion of the cell cycle synthesize substantial amounts of D-2 in vitro while the synthesis of D-2 was not detected by rough thylakoids isolated early in the light period or midway through the dark portion of the cell cycle. The same result was found when the synthesis of D-2 was investigated in vivo by pulse-labeling synchronous cell cultures with [³H]arginine. This provides further evidence for the faithful synthesis of polypeptide D-2 in vitro. These results show that one function of thylakoid-bound ribosomes is to synthesize thylakoid membrane proteins. The results also show the cell cycle regulation of membrane protein synthesis that exists within the chloroplast on thylakoid-bound ribosomes.