Packed with DNA
Bacteriophage phi29 has a tiny motor that pumps its DNA into the capsid (protein shell) during the process of virus assembly. The potential energy of the tightly coiled DNA may help phi29 inject its genetic material into the bacterial cells it infects. Scientists at the University of California, San Diego have used laser tweezers to measure the forces exerted by the motor as it pushes the DNA into the capsid. The virus’ motor has to do mechanical work to overcome two factors that create resistance. First, the DNA must be forced to bend. Second, the electrostatic repulsion of the DNA’s negatively charged phosphate backbone must be overcome. Including positively charged ions into the capsid are the solution are critical to overcoming charge repulsion. Without the right combination of positively charged ions, the virus cannot force all of its DNA into the capsid.
