Abstract
A type B Salmonella phage P221, derived from recombination between a type A phage P22 and a type B phage Fels 1, carries the protein coat of Fels 1 and the P22 early genes, at least the c to h21 genes. One of the P221 strains, P221dis, is dismune over P221 lysogens and co-immune with P22. Thus it carries the Im gene (the second immunity region) of P22 to establish co-immunity with P22. Since the att region and a1 gene for somatic 0--1 antigen conversion of P22 are located between the Im and c genes, the P221dis prophage attachment site and 0--1 antigen of P221dis lysogens were analysed. P221dis prophage is integrated at the attP22 site near the pro A region of the bacterial chromosomes and expresses the somatic 0--1 antigen.