Recombinant Enterobacteria phage lambda Large terminase protein (A), partial

The terminase large subunit acts as an ATP driven molecular motor necessary for viral DNA translocation into empty capsids and as an endonuclease that cuts the viral genome from the concetamer to initiate and to end the packaging reaction. 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 (binding to packaging sequence cos), and a large terminase subunit possessing endonucleolytic, ATPase and helicase activities (DNA maturation and packaging). The terminase binds cooperatively with the host factor IHFA/IHFB to the cos site at the junction of adjacent viral genomes. The endonuclease activity cleaves the viral DNA generating 5'overhangs of 12 bp in length. The helicase activity separates the cohesive ends generating the single-stranded 'sticky' ends of the mature genome. IHFA/IHFB is also necessary for the strand separation activity of the terminase. The terminase remains bound to the left end of the genome to be packaged, forming a stable DNA-terminase complex. In a reaction facilitated by the viral assembly catalyst gpFI, the DNA-terminase complex binds to the portal of the procapsid thereby activating the translocase activity of the terminase. The terminase packages the viral DNA into the procapsid until the next cos site on the concatemer reaches the complex. The downstream cos site is then cut generating the mature right end of the genome, the heterotrimer undocks from the DNA-filled head and remains bound to the left end of concatemer's next genome.