“…As mentioned above, mutH, mutL, and mutS loci were initially identified on the basis of the mutator phenotype that they confer to the cells, and mutU was identified by its UV sensitivity (44). The spontaneous mutation rates of otherwise isogenic strains carrying mutations in mutH, mutL, mutS, and mutU were examined in a forward mutation assay in E. coli (92,225) and in Salmonella typhimurium (247). When compared with the wild type, these strains exhibited, for example, a 10 (mutU)to 165 (mutL)-fold increase in mutation rate to streptomycin resistance, and a 60 (mutU)to 1,300 (mutH)-fold increased mutation rate to valine resistance (92).…”