T4-induced activity required for specific cleavage of a bacteriophage protein in vitro

We have examined the acid-soluble products formed during incubation of labeled substrate protein from T4-infected cells with unlabeled phage-infected cell extracts. If the substrate protein is prepared from cells infected with a T4 mutant blocked in cleavage of phage head precursor proteins, the products formed in vitro include a peptide indistinguishable by several criteria from one of the T4 internal peptides. Denatured as well as undenatured protein can serve as the substrate for the formation of this peptide. As expected, this peptide is not formed if protein from either uninfected cells or cells infected with wild-type T4 is used as substrate. The formation of this peptide in vitro is dependent on a factor present in extracts of phage-infected cells but absent from extracts of uninfected cells.