Forum 12/15/2006

“Internal DNA pressure effect on the phage capsid stability. Evolutionary optimization of phage.”

Alex Evilevitch – Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, Lund, Sweden

dsDNA in many bacteriophages is highly stressed and exerts internal pressures on the capsid walls of tens of atmospheres. We investigate correlation between packaged DNA length in phage lambda [with 78 – 100 % of wild-type (wt) DNA] and capsid strength in response to nano-indentation with an Atomic Force Microscope tip. We found, that the force exerted on the capsid walls by “pressurized” wt DNA inside is equal to the maximum force that an empty capsid can withstand before it breaks. Furthermore, this internal DNA-force provides extra support for the capsid making it two times stronger compared to an empty capsid. However, this DNA-force becomes significant only for wt phages, while all shorter genome phage mutants are as fragile as empty phage. This is despite the fact that also shorter genome mutants have internal genome pressures of many atmospheres. With DNA osmotic pressure data and a simple analytical model we explain this counterintuitive behavior. It is explained due to the fact that only at wt-DNA packaging densities in phage, the osmotic pressure in the capsid, which increases monotonically with the packaged genome length, becomes sufficient to provide extra support inside the capsid by DNA hydrating water molecules. We also show, that with addition of polyvalent salt ions, this internal DNA pressure support is reduced, making the wt phage capsid strength equal to an empty capsid. Such evolutionary energy optimization by the nature presumably selects wt phages, which can survive twice as high external mechanical stress, present in the nature, compared to their mutants. This is the first experimental demonstration of the hypothesis that the size of the genome must be consistent with the size of the viral shell.

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