期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1980
卷号：77
期号：6
页码：3159-3163
DOI：10.1073/pnas.77.6.3159
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：We report the sequences of Neurospora crassa mitochondrial alanine, leucine1, leucine2, threonine, tryptophan, and valine tRNAs. On the basis of the anticodon sequences of these tRNAs and of a glutamine tRNA, whose sequence analysis is nearly complete, we infer the following: (i) The N. crassa mitochondrial tRNA species for alanine, leucine2, threonine, and valine, amino acids that belong to four-codon families (GCN, CUN, ACN, and GUN, respectively; N = U, C, A, or G) all contain an unmodified U in the first 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 we have not detected any other isoacceptor tRNAs for these amino acids suggest that N. crassa mitochondrial tRNAs containing U in the first position of the anticodon are capable of reading all four codons of a four-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 tRNAs 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. (ii) The anticodon sequence of the N. crassa mitochondrial tryptophan tRNA is U*CA and not CCA or CmCA as is the case with tryptophan tRNAs from prokaryotes or from eukaryotic cytoplasm. Because a tRNA with U*CA in the anti-codon would be expected to read the codon UGA, as well as the normal tryptophan codon UGG, this suggests that in N. crassa mitochondria, as in yeast and in human mitochondria, UGA is a codon for tryptophan and not a signal for chain termination. (iii) The anticodon sequences of the two leucine tRNAs 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. & Tzagoloff, A. (1979) Cell 18, 47-53] in which the CUA leucine codon and possibly the entire CUN family of leucine codons may be translated as threonine.
