Translational accuracy and sexual differentiation in Chlamydomonas reinhardtii

A renewal of ribosomes has been previously reported to occur during gametogenesis in C. reinhardtii. In order to further characterize these new ribosomes, we performed pulse-labelling experiments on whole cells of C. reinhardtii, during gametogenesis and in the presence of various aminoglycosides known to alter translational accuracy: Hygromycin and Paromomycin are assumed to increase the rate of translational errors at the level of 80S and 70S ribosomes whereas Kasugamycin is assumed to induce the opposite effect. Three lines of evidence support an increased inaccuracy in protein translation during gametogenesis: (1) gamete cells displayed a higher sensitivity than vegetative cells to Hygromycin and Paromomycin; 4 micrograms/ml Hygromycin cancelled cytoplasmic protein synthesis in gametes but not in vegetative cells; Paromomycin induced the synthesis of new polypeptides of high molecular weight and of nuclear origin in gametes but not in vegetative cells. In addition, chloroplast protein synthesis was more sensitive to Hygromycin and Paromomycin in gametes than in vegetative cells. (2) Kasugamycin-sensitive alterations of thylakoid membranes were detected during gametogenesis. (3) 35S-misincorporation in the OEE3 polypeptide, of nuclear origin and normally devoid of sulphur containing amino acids, was more than three times higher in gametes than in vegetative cells. This increase was prevented by Kasugamycin, suggesting that 80S translation in gametes was more inaccurate than in vegetative cells. The possible significance of these changes occurring during gametogenic differentiation is discussed in light of the importance of a modulation of translational accuracy at particular stages of the life cycle in other lower eukaryotes.