Inhibition of modification and restriction for phages lambda and T-1 by co-infecting T3

Summary The host controlled modifications of phage -DNA byEscherichia coli B, K, and C (P1) can be suppressed by preinfecting the bacteria with UV-irradiated phage T3. Since UV-irradiated T3 induces an enzyme which cleaves S-adenosylmethionine into homoserine and thiomethyl adenosine, and since S-adenosylmethionine is the only methyl group donor for DNA methylation, we conclude that methylation is a required step in the host controlled modification of -DNA.T3 itself successfully infectsE. coli K and B with its nonmethylated DNA. Also, restricted phage or T1 will be accepted by the restrictive hostsE. coli B, K, and C(P1) if these are preinfected with UV-T3. It thus appears that T3 is capable of blocking the restriction mechanisms in these hosts.The inability of T3 to grow on C(P1) is not understood. Since T3-DNA is restricted but not degraded into nucleotides byE. coli C(P1) we presume that degradation is not the initial step in restriction.Supported by Grant No. GB 1033 R of the National Science Foundation.Postdoctoral fellow of the Deutsche Forschungsgemeinschaft.