Using electron microscopy and 3D computer reconstruction, UC San Diego scientists have produced the most detailed image yet of the protein envelope of an asymmetrical virus and the viral DNA packed within. By assembling over 12,000 microscopic views of frozen viral particles, the chemists have determined the structure of a bacteriophage called phi29. The image has a resolution of less than a nanometer, and will help to show how the virus locks onto its host and infects the cells by injecting its DNA.
A comparison between images of the virus with and without its DNA cargo revealed that the DNA twists tightly into a donut shape, or toroid, in the neck of the virus between its head and tail. "This highly distorted DNA structure is unlike anything previously seen or even predicted in a virus," says Timothy Baker, who headed the research team. The knot-like shape of the toroid, along with interlocking bumps in the protein envelope, may keep the DNA wedged into the capsid until the virus docks onto the host cell.