Suppression of a nuclear aep2 mutation in Saccharomyces cerevisiae by a base substitution in the 5 '-untranslated region of the mitochondrial oli1 gene encoding subunit 9 of ATP synthase

Mutations in the nuclear AEP2 gene of Saccharomyces generate greatly reduce d levels of the mature form of mitochondrial oli1 mRNA, encoding subunit 9 of mitochondrial ATP synthase. A series of mutants was isolated in which th e temperature-sensitive phenotype resulting from the aep2-ts1 mutation was suppressed. Three strains were classified as containing a mitochondrial sup pressor: these lost the ability to suppress aep2-ts1 when their mitochondri al genome was replaced with wild-type mitochondrial DNA (mtDNA). Many other isolates were classified as containing dominant nuclear suppressors. The t hree mitochondrion-encoded suppressors were localized to the oli1 region of mtDNA using rho(-) genetic mapping techniques coupled with PCR analysis; D NA sequencing revealed, in each case, a T-to-C nucleolide transition in mtD NA 16 nucleotides upstream of the oli1 reading frame, It is inferred that t he suppressing mutation in the 5' untranslated region of oli1 mRNA restores subunit 9 biosynthesis by accommodating the modified structure of Aep2p ge nerated by the aep2-ts1 mutation (shown here to cause the substitution of p roline for leucine at residue 413 of Aep2p). This mode of mitochondrial sup pression is contrasted with that mediated by heteroplasmic rearranged rho(- ) mtDNA genomes bypassing the participation of a nuclear gene product in ex pression of a particular mitochondrial gene. In the present study, direct R NA-protein interactions are likely to form the basis of suppression.