[Methylation of DNA of 1P+F phage for Bacillus brevis var. G-B]

DNA from a virulent mutant of the temperate 1P+f phage for Bacillus brevis var. G-B belongs to AT-type (GCequals34.5 mole %) and contains 5-methylcytosine (0.17 mole %) and N6-methyladenine (0.32 mole %). The amount of these minor bases in the phage DNA does not depend on the bacterial host (P- and S-variants of Bac.brevis var. G.-B). In contrast to the host DNA and heterologous DNA from Pseudomonas aeruginosa cells, DNA of 1P+f phage does not accept methyl groups from (methyl-3H)-S-adenosyl methionine on being methylated in vitro by enzymes from Bac. brevts cells. Thus, these phage DNA are fully methylated in vivo. The character of methylation of phage DNA in the cells of different Bac. brevis var. G-B variants is the same, i.e. dissociation of Bac. brevis var. G-B is not accompanied by any changes in the specificity of DNA methylation. In 1P+f DNA 5-methylcytosine is contained in all pyrimidine isopliths, the maximum quantity of this base (27%) is established to be present in dipyrimidine clusters. In host DNA all 5-methylcytosine localization in mono- and dipyrimidine fragments at a ratio of 1:1. This means that the specificity of methylation of cytosine residues in host and phage DNA in Bac. brevis var. G-B cells is quite different. This difference seems to be due to a specific methylase(s) induced by phage 1P+f which participates in the phage DNA methylation in vivo in addition to host enzymes.