Capstan friction model for DNA ejection from bacteriophages

Abstract

Bacteriophages infect cells by attaching to the outer membrane and injecting their DNA into the cell. The phage DNA is then transcribed by the cell's transcription machinery. A number of physical mechanisms by which DNA can be translocated from the phage capsid into the cell have been identified. A fast ejection driven by the elastic and electrostatic potential energy of the compacted DNA within the viral capsid appears to be used by most phages, at least to initiate infection. In recent in vitro experiments, the speed of DNA translocation from a λ phage capsid has been measured as a function of ejected length over the entire duration of the event. Here, a mechanical model is proposed that is able to explain the observed dependence of exit velocity on ejected length, and that is also consistent with the accepted picture of the geometric arrangement of DNA within the viral capsid.

FIG. 1

Sketch showing the arrangement of the viral DNA in the capsid. The right panel shows a sectional view when the point of observation is on the central axis. The DNA enters the capsid through the tail opening (central circle on the right panel) and is forced to curve on encountering the wall. It then wraps around the central axis in helical coils of decreasing radius.

FIG. 2

Dependence of mobility on DNA length remaining in capsid. Measured values of m(N) from Fig. 5 in the paper by Grayson et al. [16] shown with the fitted exponential, equation (5). The data are for λcI60 in NaCl buffer (solid circle), λb221 in NaCl buffer (solid square), λcI60 in MgSO₄ buffer (open circle) and λb221 in MgSO₄ buffer (open square). Error bars are not shown.