Physico‐Chemical Properties of Mitochondrial Ribosomal RNA from Fungi

Several physico‐chemical properties of mitochondrial and cytoplasmic ribosomal RNA from the filamentous fungi, Trichoderma viride and Neurospora crassa were determined and compared with those for Aspergillus nidulans .1 The thermal denaturation patterns of the mitochondrial rRNAs differed markedly from those of their homologous cytoplasmic rRNAs. The two rRNAs, therefore, possess distinct conformations in solution. 2 The guanine + cytosine content of Trichoderma mitochondrial rRNA was shown to be very low (31.5 and 35.5% for the heavy and light components) and to differ considerably from that of its cytoplasmic rRNA (50 and 49%). Such nucleotide compositions are characteristic of fungal mitochondrial rRNA and account, at least in part, for the unusual physical characteristics which these polynucleotides exhibit. 3 Polyacrylamide gel electrophoresis of mitochondrial and cytoplasmic rRNA was performed in buffers of different ionic strenghts. In low‐salt buffer, the relative mobilities of the mitochondrial rRNAs were severely retarded, causing mitochondrial rRNA to migrate more slowly than cytoplasmic rRNA. In Mg buffer, mitochondrial and cytoplasmic rRNA co‐electrophoresed and were easily distinguished from Escherichia coli rRNA. Thus, under certain conditions, the conformational peculiarities of fungal mitochondrial rRNAs are expressed with this technique as well. It is concluded that accurate extrapolations from electrophoretic mobilities to molecular weight values cannot be made for these rRNA samples. 4 Mitochondrial rRNAs sediment more slowly than their corresponding cytoplasmic rRNAs. However, under the same experimental conditions the sedimentation velocity patterns for the various mitochondrial rRNAs are not identical. 5 While the above measurements clearly distinguish mitochondrial rRNAs, as a group, from cytoplasmic and bacterial rRNAs, they also point out a considerable physical heterogeneity among the fungal mitochondrial rRNAs.