EXCISION OF 3-METHYLGUANINE FROM ALKYLATED DNA BY 3-METHYLADENINE DNAGLYCOSYLASE-I OF ESCHERICHIA-COLI

Escherichia coli has two DNA glycosylases for repair of DNA damage cau sed by simple alkylating agents. The inducible AlkA DNA glycosylase (3 -methyladenine [m3A] DNA glycosylase II) removes several different alk ylated bases including M3A and 3-methylguanine (m3G) from DNA, whereas the constitutively expressed Tag enzyme (M3A DNA glycosylase I) has a ppeared to be specific for excision of M3A. In this communication we h ave reexamined the substrate specificity of Tag by using synthetic DNA rich in GC base pairs to facilitate detection of any possible methyl- G removal. In such DNA alkylated with [H-3]dimethyl sulphate, we found that M3G was excised from double-stranded DNA by both glycosylases, a lthough more efficiently by AlkA than by Tag. This was further confirm ed using both N-[H-3]methyl-N-nitrosourea- and [H-3]dimethyl sulphate- treated native DNA, from which Tag excised M3G with an efficiency that was about 70 times lower than for AlkA. These results can explain the previous observation that high levels of Tag expression will suppress the alkylation sensitivity of alkA mutant cells, further implying tha t m3G is formed in quantity sufficient to represent an important cytot oxic lesion if left unrepaired in cells exposed to alkylating agents.