Removal of minor methylation products 7-methyladenine and 3-methylguanine from DNA of Escherichia coli treated with dimethyl sulphate.

Persistence of methylpurines in DNA methylated in vitro and in vivo in Escherichia coli WP2 cells, by dimethyl sulphate (DMS) was studied, with particular reference to the minor products 7-methyladenine and 3-methylguanine, not previously investigated in this respect, but known to be removed from DNA in vitro by spontaneous hydrolysis at neutral pH. The half-life of 7-methyladenine in vivo was relatively short (2.6 +/- 0.2 h) but not significantly shorter than in vitro at pH 7.2, 37 degrees C. The half-life of 3-methylguanine was 3.6 +/- 0.3 h in vivo, markedly shorter than in vitro, where its stability was somewhat greater than that of 7-methylguanine. Enzymatic excision of 3-methylguanine was therefore indicated to occur in E. coli. Previous findings that 7-methylguanine is probably not enzymatically excised from DNA in vivo, whereas 3-methyladenine is rapidly removed, were confirmed, and additional support for the concept of enzymatic removal of 3-methyladenine was obtained by showing extensive inhibition of its removal from cells treated with iodoacetamide prior to methylation. It is suggested that methylations of adenine or guanine in DNA at N-3 constitute blocks to template activity of DNA and stimulate a "repair" response of enzymatic removal of 3-methylpurines. Possible valence bond structures for 3-methylpurine residues in DNA are discussed, leading to the suggestion that ionized forms with positively charged amino groups may be the most effective blocks to template activity.