PETl 11 , a Saccharomyces cerevisiae Nuclear Gene Required for Translation of the Mitochondrial mRNA Encoding Cytochrome c Oxidase Subunit I 1

Mutations in the nuclear gene PETl I I are recessive and specifically block accumulation of cytochrome c oxidase subunit I1 (coxII), the product of a mitochondrial gene. However, the cox11 mRNA is present in p e t l I I mutants at a level approximately one-third that of wild type. The simplest explanation for this phenotype is that PETl I I is required for translation of the cox11 mRNA. The reduced steady-state level of this mRNA is probably a secondary effect, caused by increased degradation of the untranslated transcript. Mitochondrial suppressors of petl I I, carried on rhomtDNAs, bypass the requirement for PETl 1 I in cox11 translation. Three suppressors are fusions between the cox11 structural gene and other mitochondrial genes, that encode chimeric proteins consisting of the N-terminal portions of other mitochondrially coded proteins fused to the cox11 precursor protein. When present together with rho+ mtDNA in a heteroplasmic state, these suppressors allow cox11 synthesis in petl I I mutants. Thus in wild type, the PETl 11 product, or something under its control, probably acts at a site coded in the proximal portion of the gene for cox11 to promote translation of the mRNA. PETl I 1 was isolated by molecular cloning and genetically mapped to a position approximately midway between rnal and SUP8 on chromosome XIII. EAST mitochondrial genes are expressed by an Y organellar genetic system whose protein components are virtually all coded by nuclear genes (DuJON 1981). Interestingly, a number of recessive nuclear mutations that affect mitochondrial gene expression do not generally inactivate the mitochondrial genetic system, but rather block expression of specific mitochondrial genes. These positive activators of specific mitochondrial gene expression have thus far all been found to act post-transcriptionally (Fox 1986). T h e products of two yeast nuclear genes, PET494 and PET54, have been shown to activate translation of the mitochondrial mRNA encoding cytochrome c oxidase subunit I11 (~0x111) (MULLER et al. 1984; COSTANZO and Fox 1986; Fox 1986; COSTANZO, SEAVER and FOX 1986) while two other nuclear genes, CBPG (DIECKMANN and TZACOLOFF 1985) and CBSl (formerly MK2: RODEL, KORTE and KAUDEWITZ 1985; RODEL and FOX 1987) are required for translation of the mRNA encoding apocytochrome b . Thus there appears to be a group of nuclear genes in yeast that control the translation of specific mitochondrial mRNAs. In this study we have investigated the nuclear gene defined by p e t l I 1 mutations, originally characterized as being deficient in cytochrome c oxidase [formerly termed p e t E I I : EBNER, MASON and SCHATZ (1973)l. Subsequent work indicated that a p e t l 11-1 mutant ' Present address: ARC0 Plant Cell Research Institute, 6560 Trinity Court, Dublin, CA 94566. Genetics 115 637-647 (April, 1987) specifically lacked cytochrome c oxidase subunit I1 (~0x11) (CABRAL and SCHATZ 1978), the product of the intronless mitochondrial gene a x i l (CABRAL et al. 1978; Fox 197913; CORUZZI and TZAGOLOFF 1979). T h e pathway of expression of axil to yield cox11 is of particular interest since cox11 is the only known yeast mitochondrial gene product to be translated as a precursor-protein that is proteolytically processed to mature form (PRATJE et al. 1983). Our examination of the phenotype of pet1 1 I mutants and the nature of mitochondrial mutations that suppress p e t l 1 I, strongly suggests that the wild-type PETl I I gene product is required specifically to activate translation of the mRNA encoding ~0x11. Thus the PETIII gene product appears to belong to the group of nuclearly coded regulators of mitochondrial translation. To facilitate further studies on PETl 1 I we have isolated the gene on a 2.7 kilobase (kb) restriction fragment of yeast DNA and mapped its chromosomal location to a position midway between mal and SUP8 on chromosome XZZZ. MATERIALS AND METHODS Media, genetic methods and yeast strains: YPEG medium contained 1% (w/v) yeast extract, 2% (w/v) peptone, 3% (v/v) ethanol and 3% (v/v) glycerol. YPGal was the same as YPEG except that it contained 2% (w/v) galactose instead of ethanol and glycerol. Glucose-containing medium (YPD), minimal medium (SD) and genetic techniques were as described (SHERMAN, FINK and LAWRENCE 1974). The wild-type D273-10B (ATCC 25657) and DAlrho' 638 C. G. Poutre and T. D. Fox