  A variety of viruses tightly pack their genetic material into protein capsids
that are barely large enough to enclose the genome. In particular, in
bacteriophages, forces as high as 60 pN are encountered during packaging and
ejection, produced by DNA bending elasticity and self-interactions. The high
forces are believed to be important for the ejection process, though the extent
of their involvement is not yet clear. As a result, there is a need for
quantitative models and experiments that reveal the nature of the forces
relevant to DNA ejection. Here we report measurements of the ejection forces
for two different mutants of bacteriophage lambda, lambda b221cI26 and lambda
cI60, which differ in genome length by ~30%. As expected for a force-driven
ejection mechanism, the osmotic pressure at which DNA release is completely
inhibited varies with the genome length: we find inhibition pressures of 15 atm
and 25 atm, respectively, values that are in agreement with our theoretical
calculations.
