A SIMPLE ASSAY FOR MONITORING THE MUTAGENIC EFFECTS OF 5-METHYLCYTOSINE DEAMINATION IN ESCHERICHIA-COLI

We have developed and tested a simple phenotypic assay which monitors C to T transition mutations at the second C of a CCAGG sequence in the lacZ gene of Escherichia coli. The assay is based on new data concern ing amino acid requirements on either side of a crucial active site re sidue in beta-galactosidase, glutamic acid 461. We show that the frequ ency of occurrence of the mutation is influenced by two genes: dcm, th e cytosine methylase gene, and vsr, one of the genes involved in very short patch repair. The assay has been used to evaluate the function o f vsr cloned from a potential very short patch repair mutant.