Organization of tRNA and rRNA genes in N. crassa mitochondria: intervening sequence in the large rRNA gene and strand distribution of the RNA genes.

Through analysis of cloned fragments of N. crassa mitochondrial DNA, we have derived a physical map for the region of the mitochondrial genome which encodes the ribosomal RNAs and most of the tRNAs. We have located RNA genes on this map by hybridization of purified 32P end-labeled RNA probes, and our findings are as follows. First, the gene for the large ribosomal RNA contains an intervening sequence of approximately 2000 bp. Second, the genes for the small and large ribosomal RNAs are not adjacent, as previously reported, and the region between them contains a number of tRNA genes, including that for the mitochondrial tRNATyr, which is located close to the small rRNA gene on the same strand of the mitochondrial DNA. Third, there is a second cluster of tRNA genes on the mitochondrial DNA following the large ribosomal RNA gene, but there is no evidence for the presence of tRNA genes in the intervening sequence of the large ribosomal RNA. Fourth, hybridization of labeled ribosomal and transfer RNAs to the separated strands of a cloned 16 kbp DNA fragment covering this region indicates that the two ribosomal RNAs and most, if not all, of the mitochondrial tRNAs are encoded on one strand of the mitochondrial DNA.