Forced Bacteriophage Uncorking: Viral DNA Ejection Triggered by a Sensitive Mechanical Switch

Abstract

The foremost event of bacteriophage infection is the ejection of genomic DNA into the host bacterium after virus binding to surface receptor sites. The DNA ejection triggering mechanism is yet largely unknown. We investigated the role of mechanical force in triggering DNA ejection of the T7 bacteriophage. Mature phages containing tightly packed 40 kbp dsDNA were studied. We employed atomic force microscopy (AFM) to visualize the topographical structure and mechanically-induced changes of the phage particles covalently attached to mica surface. The sample was gently tapped so as to prevent the dislodging or fracture of the T7 capsids. We found that tapping the capsid wall with an oscillating AFM cantilever triggered a rapid DNA ejection via the tail complex. The forces employed were calculated to cause a very small (0.1-0.2%) change in the internal pressure of the capsid. Triggering rate increased exponentially as a function of force, following transition-state kinetics with an activation barrier of 23 kcal/mol at 1.2 nm along the reaction coordinate. The configuration of the ejected DNA molecule revealed that it had been exposed to a propulsive force. This propulsive force, which is thought to arise from intra-capsid pressure, assists in initiating the ejection process and the transfer of DNA across spatial dimensions beyond that of the virion. Furthermore, the internal pressure likely contributes to mechanically pre-loading the ejection machinery, thereby tilting the energy landscape towards DNA ejection; in this state a small additional, external force is sufficient to trigger the process with an apparently switch-like mechanism.