pac sites are indispensable for in vivo packaging of DNA by phage P22.

F' pro+ plasmids were selected and used as donors to prepare P22 transducing phages. Two types of result were observed. pro+ from type I donors cannot be packaged by wild-type P22 to yield transducing particles unless a prophage pac site is introduced into the plasmid. Transposon Tn10 also allows initiation of packaging. pro+ from type II plasmids can be transduced with the same efficiency as pro+ DNA on the chromosome, indicating that a chromosomal pac site was included when the F' pro+ was excised from the Hfr strain. The usefulness of type I plasmids as a test substrate for pac signals is discussed.