Isolation and Characterization of Cytoplasmic and Chloroplastic Ribosomes and Their Ribosomal RNAs from the Diatom Cylindrotheca Fusiformis

The cytoplasmic and chloroplast ribosomes from the marine diatom Cylindrotheca fusiformis were isolated and characterized. The cytoplasmic ribosomes sedimented in sucrose at 84S and dissociated into subunits of 64S and 42S in the absence of Mg2+. It contained ribosomal RNAs with molecular weights of 1.31 X 10(6) and 0.70 X 10(6). The chloroplast ribosomes sedimented at 70S only in the presence of high Mg2+ concentrations (25-100 mM). No stable subunits were routinely observed and at very high levels of Mg2+ (greater than 100 mM) the 70S species was converted to a form sedimenting at 55S. At 4 degrees C ribosomal RNAs with molecular weights of 1.1 X 10(6) and 0.40 X 10(6) were detected on polyacrylamide gel electrophoresis. When the RNAs were resolved at room temperature the large molecular weight component disappeared while RNA with molecular weights of 0.65 X 10(6) and 0.53 X 10(6) were observed. Apparently the large chloroplast RNAs dissociated into two pieces of unequal molecular weight. These properties of the diatom's chloroplast ribosomes are very similar to those of the counterparts in unicellular green algae, which suggests that both types of algae have a common phylogenetic ancestor.