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For many double stranded (ds) DNA viruses the viral genome is actively packaged into the virus capsid by a powerful molecular motor. This results in an energetically unfavorable state of the packaged genome, creating tens of atmospheres internal pressure inside the virion. Using a new calorimetric assay to determine the activation energy barriers associated with genome loss in response to elevated temperatures, we found a direct influence of internal DNA pressure on the stability of the viral particle. The Cryo EM reconstructions of bacteriophage λ (red) and Herpes Simplex 1 virus, HSV-1 (blue) are shown.
The image shows phage λ bound to its maltoporin receptor LamB on the E..coli membrane during DNA ejection. The capsids show the gradual progression of DNA disordering. This demonstrates a remarkable physical adaptation of bacterial viruses to the environment of E. coli cells in a human host.

Image by: Ting Liu

The genomes of double-stranded DNA viruses are packaged to high densities within thin protein shells. The Cryo EM reconstructions of three bacteriophage lambda particles shown are empty, partially and completely packaged with viral genomes. Left particle contains 100% of the wild-type lambda genome with resulting DNA - DNA surface separations corresponding to only 6 angstroms.

Image by: Gabriel Lander

In the assembly of many viruses, double-stranded DNA is packaged into a preformed capsid by the action of portal molecular motor complex. Shown are the structures of the empty (right) and filled (left) capsids of bacteriophage lambda as determined by cryo-electron microscopy and 3D image reconstruction by Gabriel Lander and John Johnson (The Scripps Research Institute) and Alex Evilevitch (Lund University)
In this issue: Viral replication can begin only after a virus successfully ejects its genome into a host cell. Within capsids, viral DNA is arranged as a hexagonal, crystalline structure with little mobility due to friction between DNA strands. The authors observed, however, that the internal energy of DNA within the capsid of bacteriophage λ transitioned abruptly at around 33 °C due to a sudden decrease in the amount of ordered DNA in the capsid. Increased disorder, along with decreased DNA density, reduced friction between strands and enhanced genome mobility, according to the authors.