Osmotic pressure: resisting or promoting DNA ejection from phage

TL;DR

This study combines experiments and modeling to show that osmotic pressure can promote DNA ejection from bacteriophages in crowded environments, challenging previous assumptions that it resists ejection.

Contribution

It reveals that osmotic stress enhances phage DNA ejection in vivo, contrary to prior in vitro findings, and identifies optimal crowding conditions for DNA release.

Findings

Osmotic pressure promotes DNA ejection in crowded environments.

DNA-binding proteins increase ejection extent under crowding.

Optimal crowding conditions maximize DNA remaining in capsid.

Abstract

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.