Each particle of the p22 bacteriophage, a herpes cousin, contains a single copy of its genome. The DNA is pumped as a long concatamer into one end of the capsid through a portal of gp1 proteins. After one genome enters, the concatamer is cleaved and the portal plugged shut. In gp1 mutants, too much DNA is let in, but just how the wild-type portal senses full capacity was unknown.
Johnson's group viewed the fully assembled portal using automated electron microscopy, which identified items of interest systematically, thus retrieving ten times as many images as in manual reconstructions. The higher resolution data revealed features of the intact viral particle that were previously hidden, including the gp1 portal. “Everything we dreamed of seeing was staring us in the face,” said Johnson.
The portal had previously been seen as an isolated entity, but it looked much different in the DNA-filled virion. A ring of DNA wrapped around the portal “sort of like a belt,” said Johnson. “It looks like the DNA is squeezing the portal and changing its conformation. This probably signals to the outside, ‘we are full of DNA.’” The pumps then know to cut the DNA.