Bacteriophage P2-infected cell extracts lacking the M gene product can package linear (mature) P2 DNA, but not its closed circular precursor, into phage particles. Thus the P2 M gene product is required for site-specific DNA cleavage (ter cleavage) during DNA packaging. The P2 M and P gene products have been extensively purified using an in vitro complementation-packaging assay. When studied in vitro, the site-specific DNA cutting which generates mature P2 DNA requires P2 head structures in addition to purified M and P proteins, spermidine, and ATP. © 1979.