Studies on the structure and cellular location of various ribosome and ribosomal RNA species in the green alga Chlamydomonas reinhardi

Under ionic conditions effecting little or no subunit dissociation, Chlamydomonas reinhardi contains 2 major classes of ribosomes with generic sedimentation velocities of 83 and 70s and 3 minor classes with sedimentation velocities of 66, 54, and 41 s. Ribosomal RNAs with sedimentation velocities of 25, 23, 18, 16 and 5 s have been identified. The 70-sribosomes are in the chloroplast and contain 23-, 16- and 5-s ribosomal RNA whereas the 83-s ribosomes are in the cytoplasm and contain 25-, 18- and 5-s ribosomal RNA. Numbers of chloroplast ribosome particles counted in electron micrographs of wild type C. reinhardi and the ac-20 and y mutants have been compared with relative amounts of 70-s ribosomes determined by sucrose gradient sedimentation and amounts of 23-, 16-and 5-s ribosomal RNA determined by gel electrophoresis. In response to reduced concentrations of magnesium the 70-s ribosomes of wild type are susceptible to a progressive reduction in sedimentation velocity whereas the 66-s ribosomes of the mutant ac-20 are not. Chlorophyll synthesis and the formation of the chloroplast lamellar system do not appear to be correlated with the relative amounts of chloroplast ribosomes.