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The T7 bacteriophage contains a linear molecule of double-stranded DNA packaged within a proteinaceous capsid. The capsid is thought to play a mechanoprotective role and to store the energy necessary for DNA injection into host bacterium. In previous melting experiments on T7 particles followed with UV- and CD- spectroscopy two thermal transition steps were detected: one around 65 °C and another around 80 °C. These transitions are probably due to the loosening of the capsid structure and DNA strand separation, respectively. Conceivably, these transitions also influence the capsid's mechanical characteristics. To investigate the effect of temperature on the morphology and nanomechanical properties of the T7 bacteriophage, surface-attached particles were studied with atomic force microscopy and force spectroscopy methods. The experiments were carried out at room temperature, 65 °C and 80 °C. The surface-attached virus particles appeared in the AFM images as spherical objects with a radius of 25-30 nm, a value that correlates with cryo-electron microscopic data. During indentation of the particles we most often observed a force transition containing two subsequent peaks. Conceivaby, the first peak corresponds to the collapse of the upper shell wall, whereas the second peak to the rupture of the lower one. The stiffness of T7 phage wall calculated from the slope of the first transition increased as a function of temperature. In sum, the temperature-dependent changes related to the structural alteration of the T7 capsid's proteins and its DNA result in global changes in the capsid's nanomechanical properties. |
