Purification and DNA-binding activity of the PacA subunit of the bacteriophage P1 pacase enzyme

The bacteriophage P1 packaging site (pac) cleavage enzyme (pacase) consists of two phage encoded proteins, PacA and PacB. Both proteins are necessary for the recognition and cleavage of pac and for subsequent packaging of cleaved DNA into phage particles. We have purified PacA to homogeneity from a bacterial strain that overproduces the protein. Purified PacA complements an Escherichia coli extract containing the PacB protein for DNA cleavage at the pac site and recognizes and binds to methylated pac DNA independently of PacB in gel retardation experiments. The latter property of PacA is absolutely dependent on the presence of a wildtype E. coli extract, suggesting that E. coli host proteins play a role in the pac cleavage reaction.