Alkylation Induced DNA Repair and Mutagenesis in Escherichia coli.

This support summarizes studies on repair of methylmethane-sulfonate MMS alkylation lesions in DNA of the bacterium Escherichia coli. It shows that E. coli has two distinct 3-methyladenine M3A DNA glycosylase activities one is constitutively expressed and encoded by the tag gene TagI, whereas the other is inducible and encoded by alkA TagII. The tag glycosylase is identified radiochemically as a 21 kdal protein whereas the alkA product is a 30 kdal protein. It is induced upon exposure of the cells to low levels of alkylating agents, a treatment that induces the adaptive response. TagII is not under control of recA, necessary to induce the mutagenic SOS response. TagI appears responsible for rapid repair of m3A alkylation products in unadapted cells. The inducible enzyme, TagII, is required for killing adaptation to alkylation resistance and for repair of potentially lethal lesions not recognized by the constitutive enzyme in unadapted cells. Persisting m3A alkylation products in DNA are shown to be cytotoxic for cells but not mutagenic. It is indicated that DNA glycosylases have a direct role in mutagenesis by creating AP-sites as premutagenic lesions, processed by the SOS system. Increased mutations in tag or alkA mutants can be ascribed to more rapid induction of the SOS response by persisting 3-methylpurines. Keywords Genetics Gene repair Genes.