Methylation in Vivo of Deoxyribonucleic Acid during Induction of Bacteriophage P1 by Thymine Deprivation

Methylation of DNA in vivo during prophage induction was studied in different mutants of Escherichia coli B3, lysogenic for phage P1b, by measuring the transfer of the methyl group from methionine to DNA bases. All the incorporated label was found in 6-methylaminopurine and in thymine. In the induced cells the specific activity of 6-methylaminopurine, expressed per unit of newly synthesized adenine, is decreased in one strain and increased in others in comparison with the noninduced cells. Evidence is presented that the DNA made before induction becomes supermethylated in thymine-deprived cultures, but without correlation with the extent of induction. Further, the rate of methylation in the induced cultures is not correlated with the rate of induction. Although specific alterations in the methylation pattern involving a small number of specific bases cannot be excluded by the analytical techniques available, it is concluded that gross supermethylation of newly synthesized DNA is not necessary for induction to occur. It seems, however, that in thymineless phage induction some excess methylation of the preformed DNA can be regarded as a common event in induced cultures, although such supermethylation is shown not to be sufficient for the induction of the prophage.