Mechanically-driven ejection of viral DNA

Viruses are parasitic infectious agents with a nanoscale shell, known as the capsid, that encapsulates the genomic material. Most bacteriophage viruses invade bacteria by transferring their genome inside the host cell while leaving the capsid outside. Thus, the foremost event of bacteriophage infection is the ejection of genomic material into the host bacterium after the virus has recognized and bound to surface receptor sites. How ejection is triggered is yet unknown. We show, by manipulating individual mature T7 phage particles, that tapping the capsid wall with an oscillating atomic-force-microscope cantilever triggers rapid DNA ejection via the tail complex. Triggering rate increases exponentially as a function of force, hence follows transition-state theory, across an activation barrier of 23 kcal/mol at 1.2 nm along the reaction coordinate. The conformation of the ejected DNA molecule revealed that it had been exposed to a propulsive force. This force, arising from intra-capsid pressure, assists in initiating the ejection process and the transfer of DNA across spatial dimensions beyond that of the virion. Chemical immobilization of the tail fibers also resulted in enhanced DNA ejection, suggesting that the triggering process might involve a conformational switch that can be mechanically activated either by external forces or via the tail-fiber complex. Considering the emerging interest in artificial micro- and nanocapsules capable of triggered material release, understanding how viral DNA ejection is triggered carries important application potential. The unique features of the single-particle mechanics method employed here may be useful in uncovering the fine details of viral DNA ejection. Financing: FP7, Hungarian Office for Research, Development and Inovation