Mitochondrial ribosomal RNA from Aspergillus nidulans: Characterization of a novel molecular species

Abstract

Several criteria have been used to compare mitochondrial ribosomal RNA from the fungus, Aspergillus nidulans, with ribosomal RNA from its corresponding cytoplasm and from Escherichia coli cells. Sedimentation velocity values of mitochondrial rRNA (23.5 S; 15.5 s) resembled those of E. coli rRNA and not of the homologous cytoplasmic rRNA (26.5 s; 17.0 s). However, when samples were analyzed by polyacrylamide gel electrophoresis, mitochondrial rRNA appeared to be larger than bacterial rRNA and similar to cytoplasmic rRNA in size. An explanation for the apparent variance in molecular weight estimation by sedimentation and electrophoretic methods was sought on the basis of configurational differences among the various rRNA species. Thermal denaturation studies were carried out at 260 mμ and 280 mμ in order to assess, separately, the contribution of A.U and G.C residues to the ordered structure of the ribosomal RNA's. Mitochondrial rRNA differed from cytoplasmic and bacterial rRNA's in several features. (1) Over the temperature range 10 to 95 °C, mitochondrial rRNA showed a greater percentage change in relative absorbance at 260 mμ than at 280 mμ while cytoplasmic and E. coli rRNA samples exhibited quite the opposite pattern. Based on these data, the G + C content of the ordered regions in the RNA chain were calculated to be 27 and 32% for the heavy and light mitochondrial rRNA components, 55.5 and 51% for the corresponding cytoplasmic ones and 54.5 and 54% for the bacterial rRNA peaks. (2) Thermal denaturation mid-points occurred at considerably lower temperatures for mitochondrial rRNA ( T m260 = 47 °C; T m280 = 48.5 °C) than for cytoplasmic rRNA ( T m260 = 54.5 °C; T m280 = 61 °C) or E. coli rRNA ( T m260 = 56 °C; T m280 = 60 °C). This is interpreted on the basis of differences in the average number of G.C residues per ordered region of the respective molecules. Nucleotide base composition studies were also carried out. They showed mitochondrial rRNA to have a lower G + C content (32%) than the corresponding cytoplasmic rRNA (51%). It is concluded that Aspergillus mitochondrial rRNA represents a unique and novel molecular species differing profoundly from both cytoplasmic and bacterial rRNA types.