Processing of small subunit precursor of ribulose bisphosphate carboxylase and its assembly into whole enzyme are stromal events

Chloroplast development requires the activities of both nuclear and chloroplast genetic systems, but the precise contribution of each and the mechanism by which they interact are not understood. A prime goal is to determine the site of synthesis of each chloroplast polypeptide. Studies with isolated subcellular systems, as well as inhibitor experiments with intact cells, have established that many chloroplast polypeptides are made by cytoplasmic ribosomes. Such polypeptides include subunits of both thylakoid and stromal multi-subunit proteins. These observations raise two questions: how do these polypeptides traverse the chloroplast envelope, and where are they assembled into whole functional proteins? The small subunit of the chloroplast enzyme ribulose biphosphate carboxylase is made in the cytoplasm as a higher molecular weight polypeptide precursor. We report here that the cleavage of this precursor and the assembly of the small subunit into whole ribulose bisphosphate carboxylase molecules takes place in the chloroplast stroma.