title: Removal of minor methylation products 7-methyladenine and 3-methylguanine from DNA of Escherichia coli treated with dimethyl sulphate.
abstract: 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.
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000378:metabolism; D004269:DNA, Bacterial / Q000378:metabolism*; D004926:Escherichia coli / Q000187:drug effects / Q000378:metabolism*; D006147:Guanine / Q000031:analogs & derivatives; D013464:Sulfuric Acids / Q000494:pharmacology*
chemical_list: D004269:DNA, Bacterial; D013464:Sulfuric Acids; D006147:Guanine; D004248:DNA (Cytosine-5-)-Methyltransferases; D000225:Adenine