On the mechanism of transcription of the lambda genome during induction of lysogenic bacteria

When a thymine-requiring lysogen carrying a thermoinducible prophage (λ857) is heated at 42 °C in a medium lacking thymine and containing chloramphenicol, the capacity to synthesize phage-specific RNA is immediately stimulated. The maximum de-repression thus achieved corresponds to a 10- to 20-fold increase in the initial rate of λ RNA synthesis. If the mutation causing heat sensitivity of the prophage represser is of the C I B type (Lieb, 1966) this de-repression in RNA synthesis is partially (λt1) or fully (λ857) reversible after cooling to 34 °C, even in the presence of chloramphenicol. In contrast, if lysogens belong to the C I A type (λ tU32, λtU37) de-repression remains maximum after cooling. The λ-specific RNA fraction induced in the absence of DNA and protein synthesis (“restricted RNA”) and the one synthesized during the replicative period of phage growth have been compared for their annealing characteristics. “Restricted” λ RNA has the properties of a GC-poor polynucleotide fraction, whereas “vegetative” λ RNA behaves as a mixture of GC-poor and GC-rich polynucleotides species. Finally “restricted” λ RNA displays a close sequence homology to λ specific RNA's from mitomycin-induced lysogens carrying early defective prophages. It can thus be concluded that lysogen induction, in conditions precluding prophage replication and protein synthesis, involves, as an immediate consequence, the enhanced transcription of certain λ genes necessary for early viral functions.