Mutant alleles of the MRS2 gene of yeast nuclear DNA suppress mutations in the catalytic core of a mitochondrial group II intron.

Previous studies show that some yeast strains carrying point mutations of domain 5 that block splicing of a mitochondrial group II intron yield spontaneous revertants in which splicing is partially restored by dominant mutations of nuclear genes. Here we cloned and sequenced the suppressor allele of one such gene, and found it to be a missense mutation of the MRS2 gene (MRS2-L232F). The MRS2 gene was first implicated in group II intron splicing by the finding that overexpression of the wild-type gene weakly suppresses the splicing defect of a mutation of another intron. Tetrad analysis showed that independently isolated suppressors of two other domain 5 mutations are also allelles of the MRS2 gene and DNA sequencing identified a new missense mutation in each strain (MRS2-T230I and MRS2-L213M). All three suppressor mutations cause a temperature-sensitive respiration defect that is dominant negative in heterozygous diploids, but those strains splice the mutant intron at the elevated temperature. The three mutations are in a domain of the protein that is likely to be a helix-turn-helix region, so that effects of the mutations on protein-protein interactions may contribute to these phenotypes. These mutations suppress the splicing defect of many, but not all, of the available splicing defective mutations of aI5gamma, including mutations of several intron domains. Protein and RNA blot experiments show that the level of the protein encoded by the MRS2 gene, but not the mRNA, is elevated by these mutations. Interestingly, overexpression of the wild-type protein restores much lower levels of splicing than were obtained with similar elevated levels of the mutated Mrs2 proteins. The splicing phenotypes of these strains suggest a direct role for Mrs2 protein on group II intron splicing, but an indirect effect is not yet ruled out.