Molecular packing of nucleic acids in spherical viruses.

Electron microscopy and X-ray scattering show that double-stranded DNA is wound in a regular way in the heads of phage such as P22 and lambda. DNA is "pumped" into such heads, which may simply act as passive containers. In contrast, the protein-coat subunits of tomato bushy stunt virus have a flexibly tethered N-terminal domain that appears to be a major locus of interactions with the compactly folded RNA. Both modes of packaging permit considerable latitude in the detailed arrangement of the nucleic-acid chain, while still ensuring specific and efficient incorporation.