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.