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

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Abstract
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 41s. Ribosomal RNAs with sedimentation velocities of 25, 23, 18, 16 and 5s have been identified. The 70-s ribosomes 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.