Bacteriophage P1 carries two related sets of genes determining its host range in the invertible C segment of its genome.

The bacteriophage P1 genome carries an invertible C segment consisting of 3-kb unique sequences flanked by 0.6-kb inverted repeats. Host range mutations of P1 have been mapped in the C segment region. P1 derivatives carrying insertions and deletions in the left half of the C segment in one of two orientations termed C(+) do not affect the plaque-forming ability on Escherichia coli K12 and E coli C, whereas those having insertions in the right half of the C segment fail to form plaques on these hosts. An E. coli C mutant which allows the latter insertion mutants with the C segment in the C(-) configuration to form plaques has been isolated. Not only P1 C(-) but also P1 C(+) phages gave plaques on this E. coli C mutant. The results are consistent with the notion that the C segment of P1 carries two sets of genes for host specificity, and that C inversion alters the P1 host range through activation of one set of the genes. Furthermore, extended host range mutants can be isolated by point mutation in either set of the P1 genes. C inversion is a slow process, but it occurs on the phage genome upon its vegetative growth as well as on the prophage in the lysogenic state. The 3-kb invertible G segment of the phage Mu genome is known to be homologous with the central 3-kb part of the C segment of P1 and to carry also two sets of genes for Mu host specificity. While only Mu G(-) grows on E. coli C, both Mu G(+) and Mu G(-) phages form plaques on the E. coli C mutant sensitive to P1 C(-). In the discussion the gene organization of the P1 C segment is compared with that of the Mu G segment.