Neurospora crassa Mitochondrial tRNAs and rRNAs: Structure, Gene Organization, and DNA Sequences

Studies in this laboratory on Neurospora crassa mitochondria have included sequence analysis of tRNAs, mapping and cloning of the tRNA and rRNA genes, and the DNA sequence analysis of these genes. Results from tRNA sequence analyses have helped to explain how the mitochondrial protein-synthesizing system can function with a much smaller number of tRNAs than required by other protein-synthesizing systems. Mapping studies have shown that the two rRNA genes and almost all of the tRNA genes are clustered in one third of the mitochondrial genome. In addition, both the rRNA genes and all of the tRNA genes are coded by the same DNA strand. DNA sequence analysis has provided several interesting results. Twenty-one tRNA genes have been identified. The tRNA genes and rRNA genes are flanked by highly conserved GC-rich palindromic sequences. The intervening sequence within the large-rRNA gene has been found to contain a long open reading frame capable of coding a protein at least 258 amino acids long. The boundaries of this intervening sequence have been inferred using homology with the rRNA gene sequences of other organisms. DISCUSSION Codon Reading Patterns in Mitochondria as Deduced from Mitochondrial tRNA Sequences Analyses of the N. crassa mitochondrial tRNA sequences for alanine, leucine 2, threonine, and valine have shown that all these tRNAs corresponding to amino acids with four codon families contain an unmodified U in the first or wobble position of the anticodon (Heckman et al. 1980). In contrast, tRNAs from prokaryotes or from eukaryotic cytoplasm always contain a modified...