The terminase small subunit binds to the packaging initiation site and regulates the ATPase activity of the terminase large subunit. The terminase lies at a unique vertex of the procapsid and is composed of two subunits, a small terminase subunit involved in viral DNA recognition (packaging sequence), and a large terminase subunit possessing endonucleolytic and ATPase activities. Both terminase subunits heterooligomerize and are docked on the portal protein to form the packaging machine. The terminase large subunit exhibits endonuclease activity and cleaves the viral genome concatemer once the capsid is full (headful packaging). Once the capsid is packaged with the DNA, the terminase complex is substituted by neck proteins. Homooctamer (Probable)(PubMed:9013596). Interacts with the terminase large subunit gp17; the active complex is probably heterooligomeric (PubMed:12466275).