The distribution of nascent polypeptide chains among intact polyribosomes from Chlamydomonas reinhardi

A model of polyribosome function based on tape theory has been applied to the analysis of intact polyribosomes from Chlamydomonas reinhardi. The distribution of nascent polypeptide chains found on polyribosomes does not conform to the expected pattern in which small polypeptides are synthesized on small polyribosomes and large polypeptides on correspondingly large polyribosomes. This discrepancy was revealed in the analysis of specific activity of polyribosomes (radioactivity in nascent chains per ribosome) versus polyribosome size at labeling saturation. It was found that the specific activity of small polyribosomes was higher than predicted and that of large polyribosomes was lower. This finding was validated by measuring the sizes of nascent chains from various polyribosome size classes by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The presence of large polypeptides on small polyribosomes could be partially accounted for by the synthesis of polypeptides on chloroplast (chloramphenicol-sensitive) polyribosomes. A maximum peptide interval time of 10 s was estimated from the labeling kinetics of the nascent chains of mid-sized (cytoplasmic) polyribosomes. This rate of translation is comparable to that reported in other eucaryotic cells.