Ion-dependent dynamics of DNA ejections for bacteriophage lambda

TL;DR

This study investigates how different ions influence the dynamics of DNA ejection in bacteriophage lambda, revealing the importance of ion charge and concentration in controlling ejection forces and DNA behavior.

Contribution

It systematically explores the effects of ion concentration and identity on DNA ejection dynamics, including the role of counter-ion charge and DNA looping during ejection.

Findings

Charge of counter-ions affects ejection force

Ionic concentration subtly influences frictional forces

DNA forms loops during ejection, aiding circularization

Abstract

We study the control parameters that govern the dynamics of in vitro DNA ejection in bacteriophage lambda. Past work has demonstrated that bacteriophage DNA is highly pressurized; this pressure has been hypothesized to help drive DNA ejection. Ions influence this process by screening charges on DNA; however, a systematic variation of salt concentrations to explore these effects has not been undertaken. To study the nature of the forces driving DNA ejection, we performed in vitro measurements of DNA ejection in bulk and at the single-phage level. We present measurements on the dynamics of ejection and on the self-repulsion force driving ejection. We examine the role of ion concentration and identity in both measurements, and show that the charge of counter-ions is an important control parameter. These measurements show that the frictional force acting on the ejecting DNA is subtly dependent on ionic concentrations for a given amount of DNA in the capsid. We also present evidence that phage DNA forms loops during ejection; we confirm that this effect occurs using optical tweezers. We speculate this facilitates circularization of the genome in the cytoplasm.