Single Molecule manipulations 3

As part of their infection cycle, many viruses must package their newly replicated genomes inside a protein capsid to insure its proper transport and delivery to other host cells. Bacteriophage 29 packages its 6.6mm long double-stranded DNA into a 42 nm dia. X 54 nm high capsid via a portal complex that hydrolyses ATP. This process is remarkable because entropic, electrostatic, and bending energies of the DNA must be overcome to package the DNA to near-crystalline density.

In a recent work by Dr Bustamante, optical tweezers have been used to pull on single DNA molecules as they are packaged, thus demonstrating that the portal complex is a force generating motor. They have shown that this motor can work against loads of up to ~57 picoNewtons on average, making it one of the strongest molecular motors ever reported. Interestingly, the packaging rate decreases as the prohead is filled, indicating that an internal
pressure builds up due to DNA compression. It is estimated that at the end of the packaging the capsid pressure is ~6 MegaPascals, corresponding to an internal force of ~50 pN acting on the motor.

Reference