DNA cytosine methylation and heat-induced deamination

The heat-induced conversion of 5-methylcytosine (m⁵C) residues to thymine residues and of cytosine to uracil residues in single-stranded DNA was studied. The calculated rates for deamination at 37°C and pH 7.4 were 9.5×10^–10 and 2.1×10^–10 sec^–1, respectively. N⁴-Methyldeoxycytidine, which is in the DNA of certain thermophilic bacteria, was more heat-resistant than was deoxycytidine and much more than was 5-methyldeoxycytidine. Thermophilic bacteria which contain N⁴-methylcytosine rather than m⁵C in their genomes may thereby largely avoid heat-induced mutation due to deamination, which is incurred by the many organisms that contain m⁵C in their DNA.