Recombinant Enterobacteria phage P21 Terminase small subunit (1)

Component of the terminase that processes and encapsidates viral genomes during virion assembly. The terminase is composed of two small and one large subunits. To initiate packaging, it binds a specific sequence, at the junction of adjacent viral genomes in the concatemeric DNA substrate. Next, in a reaction stimulated by the presence of proheads and ATP but not requiring ATP hydrolysis, the terminase creates two nicks at a specific site, one on each stand. Terminase then separates the cohesive ends in a reaction requiring ATP hydrolysis. The heterotrimer remains bound to the left end of the genome to be packaged, forming a stable DNA-protein complex known as complex I. In a reaction facilitated by a viral assembly catalyst, gpFI, complex I binds a prohead, a preformed head shell precursor, to form complex II. In another packaging reaction requiring ATP hydrolysis, the DNA is translocated into the prohead until the next specific site on the concatemer reaches the packaging complex. At this time the downstream specific site is cut and the heterotrimer undocks from the DNA-filled head to remain bound to the left end of concatemer's next genome. The new heterotrimer-DNA complex I binds another prohead to continue the processive, polarized packaging of viral genomes. The terminase is dependent upon host integration host factor (ihfA/ihfB) for these activities.