Biophysical and structural characterisation of the bacteriophage HK97 DNA packaging system

DNA packaging is a key step in the assembly of dsDNA viruses such as tailed bacteriophages and herpes viruses, whereby empty capsids are filled with a copy of the viral genome. The task is mediated by a DNA packaging motor, made of terminase proteins interacting with the portal vertex of a capsid. Cos phages use a defined signal recognised by the terminase machinery to mark the beginning and end of their genome in newly replicated, concatemeric DNA. For cos phages, the structures of assemblies that initiate, perform and terminate packaging are unknown. The structures of individual terminase proteins are also unknown. To further elucidate the mechanisms of DNA packaging in cos phages, a new packaging motor was assembled in vitro based on Escherichia coli bacteriophage HK97. Structural, biochemical and biophysical evidence suggests that cos, pac and φ29-like motors share a common mechanism for DNA translocation, despite their different initiation and termination behaviours.