Packaging of DNA in cauliflower mosaic virus and bacteriophage SD studied with magnetic birefringence

Magnetic birefringence measurements are reported on solutions of two double-stranded DNA containing viruses: icosahedral cauliflower mosaic virus (CaMV) and SD, an isometric bacteriophage with a short tail. Assuming the signal comes predominantly from the DNA, constraints can be put on its packing geometry. The anisotropy of CaMV is consistent with the DNA having a near orthogonally wound spool-like structure. The DNA anisotropy is smaller in SD but is compatible with a spool-like structure if the windings are tilted about the spool axis or if a significant proportion of the DNA is effectively isotropically packed. Intermediate structures are also possible.