TABLE 4.

Effects on viability and mutation rates in haploid strains that are double mutants for deficiency in nuclease and/or MMR activities

Mutation	Effect on viability and mutation rates when combined with:
	msh2	pol2-4	pol3-01
exo1	Epistasis	Synergy	Inviablea
pol3-01	Inviablebc	Inviabled
pol2-4	Synergyc

^aThe pol3-01 exo1 haploid double mutant is not viable. The diploid strain exhibits a synergistic hypermutable phenotype (Table 3). 

^bThe pol3-01 mutant is also inviable in combination with a strain with another MMR defect, pms1; the pol3-01 pms1 (23) and pol3-01 msh2 diploid strains are viable and exhibit a synergistic hypermutable phenotype (Table 3). 

^cThe synergistic interactions between msh2 and proofreading defects (pol2-4 and pol3-01) are observed only for short runs and base substitutions but not in the long homonucleotide runs A₁₂ and A₁₄ for pol2-4 (Table 2) or in the A₁₄ run for pol3-01 (Table 3). 

^dMorrison and Sugino (24). The pol2-4 pol3-01 haploid double mutant is not viable. The diploid strain exhibits a synergistic hypermutable phenotype.