  Recent in vitro experiments have shown that DNA ejection from bacteriophage
can be partially stopped by surrounding osmotic pressure when ejected DNA is
digested by DNase I on the course of ejection. We argue in this work by
combination of experimental techniques (osmotic suppression without DNaseI
monitored by UV absorbance, pulse-field electrophoresis, and cryo-EM
visualization) and simple scaling modeling that intact genome (i.e. undigested)
ejection in a crowded environment is, on the contrary, enhanced or eventually
complete with the help of a pulling force resulting from DNA condensation
induced by the osmotic stress itself. This demonstrates that in vivo, the
osmotically stressed cell cytoplasm will promote phage DNA ejection rather than
resisting it. The further addition of DNA-binding proteins under crowding
conditions is shown to enhance the extent of ejection. We also found some
optimal crowding conditions for which DNA content remaining in the capsid upon
ejection is maximum, which correlates well with the optimal conditions of
maximum DNA packaging efficiency into viral capsids observed almost 20 years
ago. Biological consequences of this finding are discussed.
