DNA methylation of bacterial viruses T3 and T7 by different DNA methylases in Escherichia coli K12 cells.

We have investigated the susceptibility of the genomes of the related bacteriophages T3 and T7 to the three major DNA methyltransferases (EcoK, dam, dcm) of their host, Escherichia coli K12. In vivo the EcoK host specificity enzyme only methylates the DNA of ocr- phages. This is due to an inhibition of the enzyme by the phage ocr+ gene product, which had previously been shown to be an inhibitor of the restriction endonuclease. EcoK-specific DNA methylation protects the ocr- viruses after one growth cycle on these host cells against the action of corresponding restriction endonuclease EcoK. Owing to the unique S-adenosyl-L-methionine hydrolase (sam+) activity of the T3-coded ocr+ protein, the T3 DNA is absolutely devoid of the methylated bases 6-methylaminopurine and 5-methylcytosine. In contrast to this, T7 derivatives and sam- derivatives of T3 carry a small number of about 2-4 molecules 6-methylaminopurine and 5-methylcytosine per genome. The presence of 6-methylaminopurine is due to dam methylation, though the majority of dam sites remain unmethylated. In vivo as well as in vitro the ocr+ protein has no influence on the activities of the dam and dcm methylase. The experiments gave some evidence for the existence of a second cytosine methylase in E. coli K12. Besides dam and dcm recognition sites being undermethylated, their absolute number in T3 and T7 DNAs is far below the expected value. Moreover, one of the two dcm sites present in T7 (Studier strain) is missing in our T7 strain owing to a 1300-base-pair deletion in gene 0.7.