A single base change in the extra-arm of yeast mitochondrial tyrosine tRNA affects its conformational stability and impairs aminoacylation.

The mitochondrial temperature-sensitive mutation tsm-8 maps on a 1.8 kb HpaII fragment of mitochondrial DNA (mt DNA) which contains genes for tRNA(Ala), tRNA(Ile) and tRNA(Tyr). The phenotype of this mutation is, among multiple pleiotropic defects, a temperature-induced reduction of mitochondrial translation. DNA sequencing of the HpaII fragment from the wild type and mutant tsm-8 revealed a single transversion from T to A in position 56 of the mutant tRNA(Tyr) gene. This nucleotide change disrupts a base pairing in the long extra arm of the tRNA cloverleaf. Revertants of the tsm-8 mutant restore correct base pairing in the extra arm by a second-site mutation in the tRNA(Tyr) gene. Analysis of the tRNA(Tyr) transcripts revealed that neither transcription nor processing of the tRNA is affected in the mutant. However, the base alteration destabilizes the conformation of the tRNA and affects its charging parameters. At the non-permissive temperature, the Michaelis-Menten constant of the mitochondrial tyrosyl-tRNA synthetase for the mutant tRNA is increased over 20-fold when compared to the wild-type tRNA. As a consequence, mitochondrial protein synthesis is drastically reduced at the restrictive temperature. Moreover, synthesis of apocytochrome b and of cytochrome oxidase subunit 3 is decreased relative to the other mitochondrially synthesized polypeptides.