A simple genetic system has been developed to test the effect of over-expression 
of wild-type or mutated human MutL homologue 1 (hMLH1) proteins on 
methyl-directed mismatch repair (MMR) in Escherichia coli. The system relies on 
detection of Lac(+) revertants using MMR-proficient or MMR-deficient E. coli 
strains carrying a lac +1 frameshift mutation expressing hMLH1 proteins. We 
report that expression of wild-type hMLH1 protein causes an approx. 19-fold 
increase in mutation rates. The mutator phenotype was due to the ability of 
hMLH1 protein to interact with bacterial MutL and MutS proteins, thereby 
interfering with the formation of complexes between MMR proteins and mismatched 
DNA. Conversely, expression of proteins encoded by alleles deriving from 
hereditary-non-polyposis-colon-cancer (HNPCC) families decreases mutation rates, 
depending on the specific amino acid substitutions. These effects parallel the 
MutL-and MutS-binding and ATP-binding/hydrolysis activities of the mutated 
proteins.