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Disruption of the nuclear MRS2 gene (mrs2-1 mutation) causes a strong pet- phenotype in strains with mitochondrial group II introns, and a leaky pet- phenotype in strains without group II introns. MRS3 and MRS4, the genes for two mitochondrial-solute carrier proteins, can suppress both phenotypes when present in high-copy-number plasmids. In order to search for further multicopy suppressors of the mrs2-1 mutant phenotype, an yeast genomic DNA library, MW90, was constructed in YEp351 from a strain deleted for the MRS2, MRS3 and MRS4 genes. Ten different Sau3A DNA fragments that act as multicopy suppressors of the mrs2-1 respiratory-deficient phenotype were isolated from this library. Some of the newly isolated genes suppress the pet- phenotypes of mrs2-1 cells in strains with and without mitochondrial group II introns. Other genes, however, are suppressors only for the mitochondrial intron-less strains. This supports the notion that the MRS2 gene product is bifunctional i.e., it is essential for the splicing of group II introns and is also involved in processes of mitochondrial biogenesis other than RNA splicing.

A gene bank of a yeast wild type DNA in the high copy number vector YEp13 was screened for recombinant plasmids which suppress the mitochondrial RNA splice defect exerted by mutant M1301, a -1 bp deletion in the first intron of the mitochondrial COB gene (bI1). A total of 17 recombinant plasmids with similar suppressor activity were found. Restriction mapping and cross-hybridization of the inserts revealed that these 17 plasmids contain three different inserts, all lacking any extended sequence homology. Each of the inserts, when present in high copy number, has a similar suppressor activity: high in the presence of mutation M1301 in bI1, a group II intron, and low but significant with the presence of few mutants in bI2 and bI3 of the COB gene, both of which are group I introns.