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The proportion of 5-methylcytosine (5MeCyt) and 6-methylaminopurine (N(6)-methyladenine, 6MeAde) in bacteriophage P22 DNA was analyzed as a function of the host-specificity the phage carried. In the DNA of P22 grown in strains harboring the modifying drug-resistance-transfer-factor N-3, the 5MeCyt content was at least twice that after growth in strains lacking the factor. In contrast, the 6MeAde level of P22 DNA was unaffected by the presence or absence of the factor. The 6MeAde and 5MeCyt levels were unaffected by factors 222 and N-1, which do not modify phage DNA. The 5MeCyt/6MeAde ratio was only slightly higher in the DNA of Salmonella strains that had received the N-3 factor. After transfer of the N-3 factor to Escherichia coli strain B, which normally lacks 5MeCyt, a high content of 5MeCyt is observed. We conclude that the N-3 factor controls a DNA methylase specific for cytosine residues. If the N-3 host specificity is imparted by cytosine methylation, this would be the first instance where a biological role for 5MeCyt has been elucidated.

Methylation of Cytosine Residues in DNA Controlled by a Drug Resistance Factor