Host specificity of DNA produced by Escherichia coli

Summary

Bacteria with A-specific restriction plate unmodified phage λ with an efficiency of 10^-2. One mutational event can produce restriction insensitive (sA^o) mutants of λ. These differ from the original sA form of λ by no other property than their response to A-host specificity. Two-parental phage crosses involving sA and sA^o, respectively, as non-selective marker allowed to map sA between genes cII and O. These data indicate that sA is the only site on λDNA with affinity for A-specific restriction. λDNA is thus an interesting substrate in in vitro A-specific restriction and modification. Using an assay based on the infectivity of λDNA on helper-infected bacteria, A-specific modification activity was found in partially purified sonicates of bacteria with A-host specificity. In parallel to modification, ³H-methyl label from s-adenosylmethionine, the only cofactor required for modification, was transferred to unmodified λDNA. No association of radioactivity was observed in control experiments with DNA from either modified λ·A or from aλsA^o mutant. These data suggest that A-specific modification is brought about by DNA methylation and that the sA^o mutation not only abolished the affinity for A-specific restriction, but also for A-specific modification.