Changes in polypeptide initiation and elongation rates during the cell cycle of Chlamydomonas reinhardi

Changes in the rate of protein synthesis during the cell cycle of Chlamydomonas reinhardi have been measured by determining changes in the separate rates of polypeptide chain initiation and elongation and in the rate of incorporation of a radioactive amino acid. The rate of polypeptide chain elongation, determined from the relative rates of labeling of two size classes of polyribosomes, varies only about twofold during the cell cycle. The rate of polypeptide chain initiation, determined from an analysis of the distribution of ribosomes in monoribosomes (and ribosomal subunits) and polyribosomes, varies more than 25-fold. Also, the overall rate of protein synthesis during the cell cycle varies to the same extent as the rate of chain initiation. Measurement of protein synthetic rates using incorporation of a radioactive amino acid ([3H]arginine) underestimates the actual change in the rate of protein synthesis during the cell cycle. The vast changes in the initiation rate during the cell cycle suggest a mechanism for selecting specific messenger RNAs for translation at different cell-cycle stages.