Protein synthesis by isolated Acetabularia chloroplasts. In vitro synthesis of the apoprotein of the P-700-chlorophyll alpha-protein complex (CP i).

Acetabularia chloroplasts can incorporate radioactive amino acids for up to several hours in vitro. The incorporation is sensitive to chloramphenicol and lincomycin, insensitive to cycloheximide, and completely light-dependent. At least 35 discrete labelled bands can be separated by SDS-polyacrylamide gel electrophoresis: 20--24 in the soluble fraction and 13--15 in the membrane fraction. Most of the label (80--85%) is in the membrane fraction, and 90% of that is in a polypeptide of 32 000 daltons. Chlorophyll-protein complexes were purified from in vitro labelled chloroplasts by SDS-polyacrylamide gel electrophoresis. CP I (P-700-chlorophyll alpha-protein complex) and its apoprotein were both labelled. This shows that the apoprotein is synthesized on chloroplast ribosomes, and can be integrated correctly into the thylakoid membrane in the absence of any cytoplasmic contribution. In contrast, no label was incorporated into the two polypeptides of CP II, the light-harvesting chlorophyll a/b complex.