Induction of a Defective Phage and DNA Methylation in Escherichia coli 15T-

SUMMARY Growth of Escherichia coli 15t − in a thymine-containing medium with any of several antibacterial agents (5-aminouracil, mitomycin C, nalidixic acid or hydroxyurea) increased the 6-methyladenine content of the extractable DNA and the DNA methylase activity. Similar changes occurred after transient (45 min.) thymine deprivation but not with other antibacterial agents such as penicillin, phenylethyl alcohol, novobiocin. Associated with the increases in 6-methyladenine content and DNA methylase activity was the induction of a defective bacteriophage. Similar effects were noted in the wild-type E. coli 15. Escherichia coli 15 T R − is a thymine auxotroph derived from E. coli 15t −. It was selected for its resistance to mitomycin C. Neither phage particles nor changes in DNA methylase or 6-methyladenine content were produced in 15t − R under conditions leading to such effects in 15t −. It is postulated that the rapid killing of ‘thymineless death’ in Escherichia coli 15t − is secondary to induction of a defective bacteriophage. The increases in 6-methyladenine content and DNA methylase activities that occur with thymine deprivation or treatment with the above antibacterial agents are thought to represent phage-induced changes.