title: Effect of 5-methylcytosine as a neighboring base on methylation of DNA guanine by N-methyl-N-nitrosourea.
abstract: Effects on N-methyl-N-nitrosourea (MNU) mediated methylation of the N7 position of guanine were compared in defined sequences of DNA containing cytosine or 5-methylcytosine (5mC) using a Maxam-Gilbert sequencing technique. Cytosine methylation in 5'-CpG-3' pairs within a subcloned fragment of the 5' region of the human HPRT gene was generated with SssI methylase and S-adenosylmethionine. Cytosine methylation was demonstrated by both the inhibition of DNA restriction by methylation sensitive endonucleases and the lack of cleavage at 5-methylcytosines by hydrazine. MNU-dependent methylation of the N7 position of guanine was inhibited up to 18% when 5mC was a 5' neighboring base to guanine and was inhibited up to 36% in an alternating CpG region in which both 5' and 3' neighboring bases of guanine were enzymatically altered to 5mC. It can be concluded that 5-methylcytosine has discernible effects on MNU methylation of the N7 position of specific guanine bases in DNA.
authors.affiliation: Department of Community and Environmental Medicine, University of California, Irvine 92717.
authors.forename: B H
authors.initials: BH
authors.lastname: Mathison
authors.forename: B
authors.initials: B
authors.lastname: Said
authors.forename: R C
authors.initials: RC
authors.lastname: Shank
mesh_terms: D044503:5-Methylcytosine; D001345:Autoradiography; D001483:Base Sequence; D003596:Cytosine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D004247:DNA / Q000378:metabolism*; D004249:DNA Damage / Q000187:drug effects / Q000502:physiology*; D006147:Guanine / Q000378:metabolism*; D006801:Humans; D008745:Methylation; D008770:Methylnitrosourea / Q000494:pharmacology*; D008969:Molecular Sequence Data
chemical_list: D006147:Guanine; D008770:Methylnitrosourea; D044503:5-Methylcytosine; D003596:Cytosine; D004247:DNA