| Abstract: | Over half of the chloroplast ribosomes isolated from growing cultures of Chlamydomonas reinhardtii are bound to chloroplast thylakoid membranes if completion of nascent polypeptide chains is prevented by chloramphenicol. The free chloroplast ribosomes are recovered in homogenate supernatants, and presumably originate from the chloroplast stroma. Only about 10% of these free chloroplast ribosomes are polyribosomes, even under conditions when 70% of free cytoplasm ribosomes are recovered as polyribosomes. The nonionic detergent Nonidet P-40 liberates atypical polyribosomes (Type I), from membranes, which require both ribonuclease and proteases for complete conversion to monomeric ribosomes. Thus Type I particles are held together by mRNA but are also held together by peptide bonds. These Type I polyribosomes probably are not bound to intact membrane, but might be bound to some protein-containing sub-membrane particle. The Type I polyribosomes are dissociated to ribosomal subunits by puromycin and high salt, and contained 0.2 to 1 nascent chain per ribosome. If membranes are treated with Nonidet and proteases at the same time, polyribosomes which are digested to monomeric ribosomes by ribonuclease alone (Type II) are obtained. Type II polyribosomes are smaller than Type I, and probably represent the true size distribution of polyribosomes on the membranes. At least 50% of the membrane-bound ribosomes are polyribosomes, since that much membrane bound chloroplast RNA is recovered as Type I or Type II polyribosomes. |
