Growth of 15 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 15.

15 is a thymine auxotroph derived from 15 . It was selected for its resistance to mitomycin C. Neither phage particles nor changes in DNA methylase or 6-methyladenine content were produced in 15 under conditions leading to such effects in 15 .

It is postulated that the rapid killing of ‘thymineless death’ in 15 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.


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