Optical tweezers studies of viral DNA packaging: Motor function and DNA confinement in Bacteriophages phi29, lambda, and T4

In the assembly of many viruses a powerful molecular motor translocates the genome into a pre-assembled capsid. We use optical tweezers to directly measure translocation of a single DNA molecule into the viral capsid. Improved techniques allow us to measure initiation and early stages of packaging. With phi29 the DNA terminal protein was found to cause large variations in the starting point of packaging. Removal of this protein results in terminal initiation, permitting more accurate assessment of motor function and DNA confinement forces. We investigated the role of electrostatic repulsion by varying ionic screening of the DNA. The observed trends are in accord with those theoretically expected considering counter-ion competition; however the forces are larger than expected in comparison with recent theories and DNA ejection measurements. We have recently succeeded in extending our methods to study two other phages: lambda and T4. These systems have unique structural and functional features, presenting an opportunity for comparative studies in this family of molecular motors. Initial measurements show that lambda and T4 translocate DNA several times faster than the phi29 motor, but are more sensitive to applied load.