The mechanism of restriction of bacteriophage λ in Escherichia coli strains: Demonstration of an in vivo requirement for S-adenosylmethionine

Abstract

Restriction of phage λ by certain Escherichia coli strains involves the degradation of the phage DNA in strains with a host-controlled modification system different from the one carried by the restricted phage. By infecting the restricting host strain with phage T3, prior to infection with phage λ, it was possible to inactivate the capacity of host cells to restrict phage λ. We have shown that this inactivation is specifically due to the production, in these T3-infected host cells, of an enzyme which cleaves S-adenosylmethionine (SAM). Since SAM is necessary for methylation reactions, it follows from these observations that methylation of the restricted λ DNA might be required to make it susceptible to the host-controlled restriction system. Several E. coli strains have a host-controlled restriction system, e.g., E. coli K12 and E. coli B, but also prophage P1 controls such a system. Our results show that both the E. coli K12 and B restriction systems require SAM for activity. The P1 restriction system, however, does not appear to have such a requirement.