Mitochondrial RNA in Xenopus laevis: II. Molecular weights and other physical properties of mitochondrial ribosomal and 4 s RNA

Abstract

The nucleotide composition and physical properties of the large and small mitochondrial ribosomal RNA's and of mitochondrial 4 s RNA from ovaries and eggs of Xenopus laevis are described. The G + C content of the mitochondrial RNA's is between 40 and 44%. The secondary structure of these RNA's, investigated in optical melting experiments, is much less stable than the structure of bacterial and cytoplasmic RNA's. Probably as a result of the structural peculiarities of the mitochondrial ribosomal RNA's, the electrophoretic mobilities of these RNA's depend greatly on the conditions of electrophoresis. Therefore, gel electrophoresis is not an applicable method for the determination of the molecular weights of the mitochondrial ribosomal RNA's. At cation concentrations from 0.07 to 2.5 m the sedimentation coefficient of the large mitochondrial ribosomal RNA is between 0.91 and 1.0 that of the cytoplasmic 18 s ribosomal RNA; for the small mitochondrial ribosomal RNA this ratio is between 0.66 and 0.77. To study the sedimentation properties of the denatured molecules the mitochondrial RNA's were compared to cytoplasmic and bacterial RNA's by sedimentation in dimethylsulfoxide, and after treatment with formaldehyde. These experiments led to the following molecular weight estimates for the mitochondrial RNA's: large ribosomal RNA, 5.3 × 10 5; small ribosomal RNA, 3.0 × 10 5; 4 s RNA (average), 28,000.