Figure 6.

Mutation frequencies conferred by missense substitutions at human MLH1 codon 44 (S44). Yeast strain YBT24 containing pSH91 was transformed with pMLH1_h(41-86), variants of this plasmid containing the indicated amino acid substitution, and the expression vector pMETc lacking an MLH1 gene. Mutation frequencies were determined using the standardized quantitative MMR assay as described in Materials and Methods. The mean mutation frequency ± SD of two to twelve independent cultures is shown. The cells containing the parental hybrid pMLH1_h(41-86) exhibited a mutation frequency of 2.7 × 10⁻⁴. The mutation defect (shown above each bar) for each variant and control was calculated by dividing the mutation frequency of cells expressing the variant by the mutation frequency of cells expressing parental hybrid pMLH1_h(41-86). A MLH1_h(41-86) gene containing a termination codon at position 44 is referred to as ‘S44-Term’.