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DNA in the bacteriophage

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Françoise Livolant
Amélie Leforestier
Marta de Frutos


Bacterial viruses, also called bacteriophages, are complex macromolecular assemblies. Their genome consists of a unique double-stranded DNA chain (tens of μm in length) confined at high density in the volume of the protein icosahedral capsid (typically tens of nm in diameter). In contrast with eukaryotic viruses, only DNA enter the host cell after recognition and fixation on a receptor localised at the surface of the bacteria.
Controlling DNA ejection in vitro, upon interaction of the phage with its purified bacterial receptor, allows us : i) to monitor the length of the DNA chain trapped in the capsid, ii) to modify at will the ionic environment of the molecule since the capsid is permeable to ions and water and iii) to visualize DNA individual chains inside the capsid using cryoEM. 

We focus on the organization and conformation of the DNA molecule confined, condensed and compressed inside the protein capsid : a 3D hexagonal crystal, full of defects. We analyse phase transitions occuring upon decrease of DNA concentration upon ejection of upon changes in the physico-chemical environment (temperature, osmotic pressure, ionic environment).

Thanks to this versatile system we explore the behaviour of confined single DNA chains under controlled conditions and question the relationship between genome transfer and organisation.


Upon ejection, the length of the DNA chain decreases and the crystalline structure turns liquid crystalline (first hexagonal, and then cholesteric) and isotropic until the capsid is finally empty. We are dealing with a monomolecular phase transition observed here by cryoTEM.


DNA conformational changes with increasing spermine 4+ concentration : from a coil (left) to a toroidal globule (right). The collapse transition is not a two states reaction : for subcritical concentrations of spermine, part of the DNA is condensed and organized in a toroid while the other part of the chain remains uncondensed around, with the two states coexisting along the chain.




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