Genome packaging into preformed viral procapsids is driven by powerful molecular motors that consist of small and large terminase proteins. The small terminase binds to viral DNA and regulates the large terminase activity; the latter will fill the procapsid with the viral genome.

In collaboration with CNB researcher Juan Carlos Alonso, Alfred Antson at the University of York has determined for the first time that although the small terminase is ring-shaped, the central region is not wide enough to accommodate the DNA inside. Recently published in PNAS, the small terminase crystal structure reveals that DNA wraps around the terminase complex by binding to an external motif present in each of the nine oligomers of the terminase.

Dr Alonso explains that the function of the small terminases is the "recognition of the viral DNA, its binding and the recruitment of the large terminase". Once this complex is assembled, it docks onto the procapsid and the small terminase performs its second function, the regulation of the enzymatic activity essential for the conformational changes that lead to translocation of the viral genome inside the procapsid.