The stromal protein large subunit of ribulose-1,5-bisphosphate carboxylase is translated by membrane-bound ribosomes.

Translation of the large subunit of ribulose-1,5-bisphosphate carboxylase (LSU) was investigated by labeling of isolated barley plastids with 35S]-methionine. In both chloroplasts and etioplasts, labeling of LSU was severely impaired if plastid membranes were removed from the reaction mixtures. Removal of membrane-bound polysomes with high salt or puromycin greatly decreased translation of LSU. Pulse-labeled chloroplast membranes were shown to release LSU if chased with unlabeled methionine in the presence of stroma. Immunoprecipitation detected higher amounts of labeled LSU translation intermediates associated with the membrane fraction than in the soluble fraction. We therefore conclude that, in plastids, membrane-bound polysomes are required not only for translation of membrane-intrinsic proteins but also for translation of a soluble protein.