Small Terminase Couples Viral DNA Binding to Genome-Packaging ATPase Activity

SummaryPackaging of viral genomes into empty procapsids is powered by a large DNA-packaging motor. In most viruses, this machine is composed of a large (L) and a small (S) terminase subunit complexed with a dodecamer of portal protein. Here we describe the 1.75 Å crystal structure of the bacteriophage P22 S-terminase in a nonameric conformation. The structure presents a central channel ∼23 Å in diameter, sufficiently large to accommodate hydrated B-DNA. The last 23 residues of S-terminase are essential for binding to DNA and assembly to L-terminase. Upon binding to its own DNA, S-terminase functions as a specific activator of L-terminase ATPase activity. The DNA-dependent stimulation of ATPase activity thus rationalizes the exclusive specificity of genome-packaging motors for viral DNA in the crowd of host DNA, ensuring fidelity of packaging and avoiding wasteful ATP hydrolysis. This posits a model for DNA-dependent activation of genome-packaging motors of general interest in virology.

► The small terminase subunit of phage P22 adopts a nonameric quaternary structure
► P22 small terminase lacks an N-terminal winged helix-turn-helix DNA-binding motif
► Residues (140–162) of P22 small terminase mediate binding to DNA and large terminase
► P22 small terminase functions as a dedicated DNA-dependent ATPase-activating factor