Stabilization of P2 tandem double lysogens by int mutations in the prophage

Abstract

Double lysogens of the temperate phage P2 were isolated from superinfection experiments in which either the superinfecting phage or the prophage carried, in addition to other markers, an integration defective ( int) mutation. The strains obtained were characterized for three properties: stability (frequency of segregation of single lysogens), cotransducibility of the prophages, and genotypes of the phages produced. Two types of double lysogens were found: those that were relatively stable, carried prophages that were not cotransducible and produced mostly phages with non-recombinant genotypes; and those that frequently segregated single lysogens, carried cotransducible prophages, and produced mostly phages with recombinant genotypes. The first type is similar to previously described P2 double lysogens that carry two prophages at different sites on the bacterial chromosome. The second type, however, most likely carries the two prophages at the same site in tandem arrangement. In one control experiment, apparent tandem strains carrying two int + prophages were also obtained, although these strains have properties different from the others. The prophage structure of the tandem strains was determined by analyzing the genotypes of the prophages of the singly lysogenic segregants that arise either spontaneously or after superinfection of the strains with int + phage. The results suggest that in tandem strains carrying one int + and one int prophage, the defective int gene always occupies a central position in the prophage structure. The significance of these results for the previously described “split-operon” model for regulation of integration in phage P2 is discussed.