Regional replication of the bacterial chromosome induced by derepression of prophage lambda

Summary

Derepression of prophage λ in E. coli strain K12 results in constitutive synthesis of the enzymes directed by the nearby bacterial operon, gal (escape synthesis). Phage 82 fails to cause escape synthesis despite that it lysogenizes the strain K12 at the site identical to that of λ on the host chromosome. The reason for the observed difference between 82 and λ is studied in the light of the recent finding that escape synthesis in λ-lysogen is closely associated to phage-promoted replication of bacterial chromosome contiguous to the prophage including gal operon (escape replication). Excision-defective mutants from 82, 82int or 82xis, do initiate escape synthesis, suggesting that the prophage 82 is normally excised too quickly after induction to allow sufficient escape replication. In support of this, much more DNA hybridizable to bacterial DNA contained in λgal accumulates after induction of 82int than after induction of 82. Studies with various hybrid phages between 82 and λ have suggested: 1. The occurrence of gal escape synthesis depends on the nature of the region between b2 and N in the λ map. 2. Regions of the 82 genome on both sides of the attachment site contribute independently to prevent gal escape synthesis. Implications of these results are discussed with regard to the factors involved in the prophage excision.