Figure 1.

Construction of isogenic λ-strains. Various plasmids carrying different λ S alleles (Sx) were each transformed into the E. coli lysogen carrying a prophage λ with mutations in cI (cI857) and S (Sam7) genes. After thermal induction of the lysogen, a mixture of phage progeny is produced, but only ones carrying the Sx allele (as a result of homologous recombination between prophage genome and the plasmid) can form plaques when plated on the bacterial host.