Sites of Synthesis of Chloroplast‐Membrane Proteins

The protein synthesis of isolated chloroplasts of Acetabularia has been measured by the in‐corporation of labeled amino acids into the trichloroacetic‐acid‐insoluble fraction as well as into individual membrane fractions. Chloramphenicol inhibited the incorporation into the trichloroacetic‐acid‐insoluble material only up to 60%, as did cycloheximide. The effect of the two antibiotics on the incorporation into the trichloroacetic‐acid‐insoluble material of isolated chloroplasts is additive. By the treatment of chloroplasts with EDTA most of the originally membrane‐bound proteins were dissolved and could be separated quantitatively from EDTA‐insoluble material. Only three protein components could be detected in the membrane fragments. Two of these membrane proteins were synthesized in isolated chloroplasts. The synthesis of one of these two proteins is inhibited completely by cycloheximide, while the labeling of the other protein has scarcely been affected. The synthesis of the cycloheximide‐insensitive protein is almost completely blocked by chloramphenicol, but the cycloheximide‐sensitive protein still has been labeled distinctly in the presence of chloramphenicol. It is concluded that these chloroplast proteins are synthesized on two different groups of ribosomes, both of which are present in the isolated chloroplasts. The presence of 80‐S ribosomes in this chloroplast preparation has been indicated by finding 26‐S rRNA. Thus it is probable that the cycloheximide‐sensitive particles in the chloroplast preparation are identical with 80‐S ribosomes. While in isolated‐chloroplasts only two membrane proteins have been synthesized, all three EDTA‐insoluble membrane proteins have been labeled in whole cells. It is concluded that the synthesis of the third protein takes place on a ribosome group in the cytosol which is active only in the intact cell.