Codon recognition rules in yeast mitochondria.

Abstract

The mitochondrial genome of Saccharomyces cerevisiae codes for 24 tRNAs. The nucleotide sequences of the tRNA genes suggest a unique set of rules that govern the decoding of the mitochondrial genetic code. The four codons of unmixed families are recognized by single tRNAs that always have a U in the wobble position of the anticodon. The codons of the mixed families are read by two different tRNAs. Codons terminating in a C or U are recognized by tRNAs with a G and codons terminating in a G or A are recognized by tRNAs with a U in the corresponding positions of the anticodons. There are two exceptions to these rules. In the AUN family for isoleucine and methionine, the isoleucine tRNA has a G and the methionine tRNA has a C in the wobble position. The tRNA for the arginine CGN family also has an A in the wobble position of the anticodon. It is of interest that the CGN codons have not been found in the mitochondrial genes sequenced to date. The simplified decoding system of yeast mitochondria allows all the codons to be recognized by only 24 tRNAs. Yeast mitochondria contain a complete set of tRNAs that function in the translation of a discrete number of mitochondrial messengers. During the past year, the sequences of most of the mitochondrial tRNA genes have been determined and virtually all of their anticodons are now known (1-4). Together with the codon usage in five mitochondrial genes, this information has permitted us to formulate the coding properties of this self-contained translational system.In this communication we present evidence that yeast mitochondria are capable of interpreting their genetic code with less than the 32 tRNAs required by the wobble hypothesis (5). The reduction in the number of tRNAs is achieved by the use of a single tRNA to recognize the four codons in each of the eight unmixed families.* These families are read by the "two out of three" method first proposed by Lagerkvist (6). Furthermore, because only two tRNAs are used to discriminate among the codons of the mixed families, the entire system including the formylmethionine initiator consists of 24 tRNAs. The 24 tRNAs represent the minimum number needed to interpret a degenerate code whose amino acid assignments follow the general outline of the universal code. Number of mitochondrial tRNAs in Saccharomyces cerevisiae Several approaches have been used to estimate the number of mitochondrial tRNAs in yeast. Separation of the aminoacylated tRNAs by reverse phase chromatography on RPC-5 has provided information about the number of isoaccepting species for each of the 20 amino acids. Such analyses indicate [31][32][33][34][35] chromatographically distinct tRNA species (7-9). This number includes species that arise from posttranscriptional modification and therefore is an overestimate of the actual number of tRNA genes. For example, Berlani et al. (3) have recently shown that a point mutation in mitochondrial DNA abolishes charging of the four histidine tRNAs, suggesting that all four isoacceptors are products of a single gen...

Nucleotide sequence of a yeast mitochondrial threonine‐tRNA able to decode the C—U—N leucine codons

“…The C-U-N codons are infrequently used in yeast mitochondrial genes [25]; and (ii) The mt DNA-coded proteins would not have been affected functionally by this change.…”

“…The two isoacceptors differ probably by only post-transcriptional modification(s) since: (i) Both migrate in the same spot when analysed by twodimensional gel electrophoresis (not shown); (ii) Only one threonine-tRNA gene was found in the cap region of mt DNA by sequencing [3].…”

Initiation of transcription of the yeast mitochondrial gene coding for ATPase subunit 9

“…Petite mutant DS400/A12, derived from D273-1OB, was a gift of Alexander Tzagoloff. The entire sequence of this petite has been reported (17,18).…”

Two genes for mitochondrial tyrosine transfer RNA in yeast

“…In yeast, a total of 24 mitochondrial tRNA genes has been identified [l-6] and this number may represent the complete set of mitochondrial tRNA genes [7]. According to the mitochondrial coding rules proposed in [7-91 the codons belonging to the 'mixed families' of the code table, and having a pyrimidine at the wobble position, are read by a single anticodon carrying a guanine at the corresponding position.…”

“…Thus the tyrosine codons UAC should be recognized by a single GUA anticodon. In fact, we have identified a single tyrosine tRNA gene [l-3] of which the anticodon is not known but is supposed to be GUA [7].…”