SUPPRESSION OF A MITOCHONDRIAL POINT MUTATION IN A TRANSFER-RNA GENE CAN CAST LIGHT ON THE MECHANISMS OF 3' END-PROCESSING

We used a genetic approach to study the nuclear factors involved in th e biogenesis of mitochondrial tRNAs. A point mutation in the mitochond rial tRNA(Asp) gene of Saccharomyces cerevisiae had previously been sh own to result in a temperature-sensitive respiratory-deficient phenoty pe as a result of the absence of 3' end-processing of the tRNA(Asp). A nalysis of mitochondrial revertants has shown that all revertants sequ enced have a G-A compensatory change at position 53, which restores th e hydrogen-bond with the mutated nucleotide. We then searched for nucl ear suppressors to identify the nuclear gene(s) involved in mitochondr ial tRNA 3' end-processing. One such suppressor mutation was further c haracterized: it restores tRNA(Asp) maturation and growth at 36 degree s C on glycerol medium in heterozygous diploids, but leads to a defect ive growth phenotype in haploids.