DNA Methyl Transferase Acting on O6-Methylguanine Residues in Adapted E. coli
Highly mutagenic alkylating agents such as N-methyl-N-nitrosourea (MNUA) or N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) produce three major puEine adducts in DNA: 7-methylguanine, 3-methyladenine, and O6-methylguanine (Lawley and Thatcher, 1970). These lesions are handled in different ways by living cells. The most abundant alkylation product, 7-methylguanine, is relatively harmless in that it does not cause an appreciable amount of miscoding, and in E. coli it does not seem to be actively removed by any repair pathway (Lawley and Orr, 1970). In contrast, 3-methyladenine is rapidly removed from DNA by an excision-repair process which is initiated by a speciqc DNA glycosylase that does not act on either 7-methylguanine or O6-methylguanine residues (giazuddin and Lindahl, 1978; Karran et al., 1980). The fate of O6-methyl-guanine in DNA has been less clear. While indications of active removal by a DNA repair process were obtained ten years ago by Lawley and Orr (1970), a DNA glycosylase catalyzing the release of such residues has not been found and may not exist. gurther, complete removal of the relatively large quantities of O6-methyl-guanine present in E. coli cells that have been treated with a high dose of MNNG is conspicuously less effective than the simultaneous liberation of 3-methyladenine from DNA.
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