Structure and Energetics of Encapsidated DNA in Bacteriophage HK97 Studied by Scanning Calorimetry and Cryo-electron Microscopy |
| |
Authors: | Robert L Duda Naiqian Cheng Roger W Hendrix Alasdair C Steven |
| |
Institution: | 1 Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260, USA 2 Laboratory of Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, Bethesda, MD 20892, USA 3 Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD 20892, USA 4 Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA |
| |
Abstract: | Encapsidation of duplex DNA by bacteriophages represents an extreme case of genome condensation, reaching near-crystalline concentrations of DNA. The HK97 system is well suited to study this phenomenon in view of the detailed knowledge of its capsid structure. To characterize the interactions involved, we combined calorimetry with cryo-electron microscopy and native gel electrophoresis. We found that, as in other phages, HK97 DNA is organized in coaxially wound nested shells. When DNA-filled capsids (heads) are scanned in buffer containing 1 mM Mg2+, DNA melting and capsid denaturation both contribute to the complex thermal profile between 82 °C and 96 °C. In other conditions (absence of Mg2+ and lower ionic strength), DNA melting shifts to lower temperatures and the two events are resolved. Heads release their DNA at temperatures well below the onset of DNA melting or capsid denaturation. We suggest that, on heating, the internal pressure increases, causing the DNA to exit—probably via the portal vertex-while the capsid, although largely intact, sustains local damage that leads to an earlier onset of thermal denaturation. Heads differ structurally from empty capsids in the curvature of their protein shell, a change attributable to outwards pressure exerted by the DNA. We propose that this transition is sensed by the portal that is embedded in the capsid wall, whereupon the structure of the portal and its interactions with terminase, the packaging enzyme, are altered, thus signaling that packaging is at or approaching completion. |
| |
Keywords: | DSC differential scanning calorimetry EM electron microscope(y) Tp peak temperature PRB phage resuspension buffer |
本文献已被 ScienceDirect 等数据库收录! |
|