Contrast Variation SANS Study of DNA Packaging in Bacteriophage λ

Abstract

The elegant physical structure of viruses has attracted wide-spread interests from structural biologists, biophysicists, and mathematicians. While the protein capsids of viruses posses the nearly universal quasi-equivalence of icosahedral symmetry, the configuration of viral DNA/RNA genome appears less structured and is still poorly understood. Here we report a contrast variation small angle neutron scattering (SANS) study of bacteriophage λ to probe the spatial organization of its dsDNA genome. Contrast variation SANS takes advantage of the different scattering lengths of protein and DNA, and can experimentally match the protein contrast so as to measure the DNA structure alone. Together with the known structure of the protein capsid, SANS data at different H/D ratios offer the opportunity for extensive structural modeling. Furthermore, knowledge of DNA configuration in bacteriophage λ impinges directly on the physics of pressurized DNA packaging known to drive the initial DNA ejection when infecting bacteria. Multivalent cations were used to modulate the internal DNA pressure, and the induced structural changes were measured by contrast variation SANS. We compare the various DNA packaging models proposed in the literature and discuss their consistencies and discrepancies with experimental data.