Distribution of 5-methylcytosine in pyrimidine sequences of deoxyribonucleic acids.

Abstract Several samples of DNA from different mammalian organs and from wheat germ were degraded with Burton reagent and the location of derivatives of 5-methylcytosine in separate fractions of the hydrolysate was investigated. In mammalian DNA's 5-methylcytosine was found to occur almost exclusively in the fraction of solitary pyrimidine nucleotides and in the terminal groups of polypyrimidine series carrying monoesterified phosphorus on C-3′ of deoxyribose. The ratio of 5-methylcytosine to cytosine was nearly identical in these positions. In the DNA from wheat germ the highest degree of replacement occurred in similar positions, but a fairly high amount of 5-methylcytosine was found in the sequence MpT. The relation of these findings to earlier evidence obtained by enzymic degradation of DNA is discussed. It is concluded that the replacement of cytosine by 5-methylcytosine is determined by the nature of the nucleotide attached to C-3′ of deoxycytidine. The replacement occurs with highest probability in the sequence CpG; somewhat less probable is the replacement in the sequence CpT, whereas in the sequences CpM, CpC and CpA the substitution takes place only rarely, even in preparations with high overall content of 5-methylcytosine.

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