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.

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