This second strategy is remarkable considering the enthalpic, entropic and DNA bending energies that must be overcome to package DNA to near crystalline densities within the confined space of the capsid.

This is the strategy used by some double-stranded (ds) RNA ( 1) and single-stranded (ss) DNA viruses ( 2, 3) and virtually all dsDNA viruses such as herpes virus, pox virus, adenovirus and all the tailed dsDNA bacteriophages ( 4). In a second strategy, an empty virus shell is first assembled, and the genome is then actively packaged into this pre-formed container.

This strategy is used by many enveloped RNA viruses such as alpha viruses and flaviruses. The virus capsid then assembles around the condensed genome. In one strategy, the genome first condenses, typically via interaction with virally encoded positively charged proteins. There are two basic strategies that viruses use for genome packaging. Taken together, these results provide insight into the evolution of functional transitions in viral dsDNA packaging motors.Ī fundamental step in the life cycle of any virus is encapsidation of the viral genome within a protein shell, or capsid, which protects the genome from environmental assault as the virus transits between hosts. However, the interacting residues differ from those identified by NMR titration in solution, suggesting that packaging motors undergo conformational changes to transition between initiation, translocation, and termination. Despite the loss of nuclease activity, fluorescence binding assays confirm the CTD retains its DNA binding capabilities and fitting the CTD into cryoEM density of the phi29 motor shows that the CTD directly binds DNA.

The NMR solution structure of the CTD indicates it is a vestigial nuclease domain that likely evolved from conserved nuclease domains in phage terminases. Sedimentation experiments show that the inter-domain linker in the full-length protein promotes oligomerization and thus may play a role in assembly of the functional motor. Here, we report structural and biophysical analyses of the C-terminal domain of the bacteriophage phi29 ATPase (CTD) that suggest a structural basis for these functional transitions. To ensure efficient genome encapsidation, these motors regulate functional transitions between initiation, translocation, and termination modes. Double-stranded DNA viruses use ATP-powered molecular motors to package their genomic DNA.

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