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Identification and Precipitation of the Polyribosomes in Chlamydomonas reinhardi Involved in the Synthesis of the Large Subunit of D-ribulose-1,5-bisphosphate Carboxylase ¹

^a Department of Biology, University of California, San Diego, La Jolla, California 92093

The size classes of polyribosomes involved in the synthesis of ribulose-1,5-bisphosphate carboxylase large subunit were determined by binding radioiodinated specific antibodies to polyribosomal preparations from Chlamydomonas reinhardi. Antibodies specific to the denatured large subunit and to the native enzyme bound primarily to small polyribosomes (N = two to five ribosomes). The binding of antibodies to small polyribosomes was unexpected since the large subunit is a large polypeptide (molecular weight 55,000) coded for by a corresponding large mRNA (12-14S). Control experiments showed that this unexpected pattern of antibody binding was not a result of messenger RNA degradation, "run-off" of ribosomes from polyribosomes, or adventitious binding of the completed enzyme to a selected class of polyribosomes. In addition, polyribosomes bearing nascent large subunit chains have been immunoprecipitated from small polyribosome fractions. A large RNA species that can direct the synthesis of large subunit in vitro was extracted from small polyribosomes.