Chloroplast ribosomal proteins of Chlamydomonas synthesized in the cytoplasm are made as precursors.

Abstract

Polyadenylated RNA from Chlamydomonas was translated in a cell-free rabbit reticulocyte system that employed [35S]methionine. Antibodies made to 4 chloroplast ribosomal proteins synthesized in the cytoplasm and imported into the organelle were used for indirect immunoprecipitation of the labeled translation products, which were subsequently visualized on fluorographs of SDS [sodium dodecyl sulfate] gels. The cytoplasmically synthesized chloroplast ribosomal proteins were 1st seen as precursors with apparent MW of 1000-6000 greater than their respective mature forms. Processing of the ribosomal protein precursors to mature proteins was effected by adding a postribosomal supernatant that had been extracted from cells of Chlamydomonas. In contrast to the chloroplast ribosomal proteins synthesized in the cytoplassm, 2 such proteins made within the chloroplast were synthesized in mature form in cell-free wheat germ translation systems programmed with nonpolyadenylated RNA.