Relationships between temperate phages Mu and P1

The temperate phages Mu and P1, although extremely different, share a short region of homology which includes the G region of Mu (termed the C region in P1). We show that the region of homology between Mu and P1 contains gene(s) that code for proteins responsible for the phages' host range specificity. Mu-P1 hybrids with the plaque morphology and immunity of Mu have been isolated by crossing Mu with two different P1 derivatives; these hybrids may acquire the host range of the P1 mutant used in the cross. Mu am+ recombinants were isolated after superinfection of a P1 lysogen with Mu Sam or Mu Uam mutants, showing that these two Mu genes are located in the region of homology between P1 and Mu. P1 am+recombinants are found after superinfection of a Mu lysogen with Plam33, am72, and am66, indicating that these P1 alleles are all located in the region of homology with Mu. From these crosses, P1-immune hybrids have been isolated with both P1 and Mu host range. There is partial serological cross-reactivity between P1 and Mu, and hybrid phages are in most cases inactivated by antiserum at a rate comparable to either parent or intermediate between them.