[Some peculiarities of phage DDVI-specific methylases]

The types of methylases are found in the cellular extract of Escherichia coli B, infected with phage DDVI. One of them is a cellular enzyme, which methylates adenine to form 6-methylaminopurine (6-MAP) and is repressed in the infected cell in vivo. The second type, which is not found in the non-infected cells, is specific for phage DDVI and induces the formation of 7-methylguanine (7-MG). Both enzymes recognize various sites, which accounts for the ratio 6-MAP/7-MG to vary in heterological DNAs between 2.07 in phage Sd DNA and 0.40 in phage DDII DNA. During in vitro incubation with homologous methylases phage DDVI DNA and especially phage T2 DNA are subjected to further methylation, which is probably indicative of their "undermethylation" in vivo. The DDVI-specific enzyme, similar to B-specific type, methylates DNA with a normal set of nitrogenous bases (phages Sd and DDII), as well as DNAs containing 5-oxymethylcytosine and glucose (phages T2 and DDVI). Both methylases under study use only native double-helical DNA as substrate and are strongly inhibited by S-adenosylhomocysteine. Phage DDVI Methylase is characterized by low stability.