The terminase small subunit binds to the packaging initiation site and regulates the ATPase activity of the terminase large subunit (By similarity). 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 (By similarity). Both terminase subunits heterooligomerize and are docked on the portal protein to form the packaging machine (By similarity). Homononamer; forms a ring-like structure through which genomic DNA is translocated into the capsid (By similarity). Heterodimer with the terminase large subunit; the active complex is probably heterooligomeric (By similarity). Belongs to the P23virus small terminase family.