Novel features in the genetic code and codon reading patterns in Neurospora crassa mitochondria based on sequences of six mitochondrial tRNAs

Abstract

The sequences of N. crassa mitochondrial alanine, leucine1, leucine2, threonine, tryptophan and valine tRNA are reported. On the basis of the anticodon sequences of these tRNA and of a glutamine tRNA, whose sequence analysis is nearly complete, the following are inferred: The N. crassa mitochondrial tRNA species for alanine, leucine2, threonine and valine, amino acids that belong to 4-codon families (GCN, CUN, ACN and GUN, respectively; N = U, C, A or G) all contain an unmodified U in the 1st position of the anticodon. In contrast, tRNA species for glutamine, leucine1 and tryptophan, amino acids that use codons ending in purines (CAGA, UUGA and UGGA, respectively), contain a modified U derivative in the same position. These findings and the fact that no other isoacceptor tRNA for these amino acids have been detected suggest that N. crassa mitochondrial tRNA containing U in the 1st position of the anticodon are capable of reading all 4 codons of a 4-codon family whereas those containing a modified U are restricted to reading codons ending in A or G. Such an expanded codon-reading ability of certain mitochondrial tRNA will explain how the mitochondrial protein-synthesizing system operates with a much lower number of tRNA species than do systems present in prokaryotes or in eukaryotic cytoplasm. The anticodon sequence of the N. crassa mitochondrial tryptophan tRNA is U*CA and not CCA or CmCA as is the case with tryptophan tRNA from prokaryotes or from eukaryotic cytoplasm. Because a tRNA with U*CA in the anticodon would be expected to read the codon UGA, as well as the normal tryptophan codon UGG, this suggests that in N. crassa mitochondria, UGA is a codon for tryptophan and not a signal for chain termination. The anticodon sequences of the 2 leucine tRNA indicate that N. crassa mitochondria use both families of leucine codons (UUAG and CUN; N = U, C, A, or G) for leucine, in contrast to yeast mitochondria [Li, M. and Tzagoloff, A., 1979] in which the CUA leucine codon and possibly the entire CUN family of leucine codons may be translated as threonine.