Initiation of packaging of double-stranded viral DNA involves the specific interaction of the prohead with viral DNA in a process mediated by a phage-encoded terminase protein. The terminase enzymes are usually hetero-oligomers composed of a small and a large subunit. This region is found on the large subunit and possess an endonuclease and ATPase activity that require Mg2+ and a neutral or slightly basic reaction. This region is also found in bacterial sequences [1,2].

1: Functional analysis of the terminase large subunit, G2P, of Bacillus subtilis bacteriophage SPP1. Gual A, Camacho AG, Alonso JC; J Biol Chem 2000;275:35311-35319. PMID:10930407

2: Molecular analysis of the Bacillus subtilis bacteriophage SPP1 region encompassing genes 1 to 6. The products of gene 1 and gene 2 are required for pac cleavage. Chai S, Bravo A, Luder G, Nedlin A, Trautner TA, Alonso JC; J Mol Biol 1992;224:87-102. PMID:1548711