Physics of Viral Infectivity: Energetics of Genome Ejection

Abstract: All viruses that infect bacteria, plant, or animal cells involve a genome (RNA or DNA) that is encapsidated by a rigid protein shell. After delivery of the viral genome into the host cell, new capsid proteins, which are encoded by viral DNA or RNA, are expressed and self-assembled into new viral capsids. The main objective of my research was to study the key physical factors of genome ejection that control the viral life cycle. One of the main steps in the viral life cycle is genome ejection, which is considered a physical process. It has recently been shown that the ejection of the viral genome from the phage capsid is driven by internal pressure reaching tens of atmospheres. This pressure is partially responsible for the delivery of the viral genome into the host cell, thus making it central in the infection process. In this doctoral thesis, direct measurements of the energy associated with genome ejection are presented. The viral capsid is “opened” by the LamB receptor protein in vitro, and ejection is measured using microcalorimetry. The energy of genome ejection from the viral capsid is obtained as a function of the relative packaging density of the viral genome. A DNA phase transition was observed by measuring the ejection enthalpy as a function of temperature. Recent in vitro experiments have shown that DNA ejection from the phage can be restricted by the surrounding osmotic pressure when ejected DNA is digested by DNase I during the course of ejection. The most important finding of this work was that the ejection of an intact genome (i.e. undigested) in a crowded environment is enhanced or even completed by the 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 resist it, while in vitro ejection is extremely dependent on the pressure within the virus capsid. The effect of internal pressure on the infection of a bacterial cell is unknown. A microfluidic technique was employed to monitor individual cells and determine the distribution of lysis due to infection in relation to the capsid pressure. The probability of lysis was found to decrease markedly with decreasing capsid pressure.