Demonstration of stepwise coiling of nucleoprotein in adenovirus core by application of critical point drying.

Mild destruction of a virus particle to observe the organized structure of the nucleoprotein complex in a virion was achieved by application of the critical point drying method. Adenovirus type 12 (ad12) virions have been treated by this method after the particles had been fixed with glutaraldehyde on an electron microscope grid. With 15 min prefixation, the capsids (shells) and the cores were in various stages of unfolding. The core was unfolded in the filamentous structure. The thickness of these filaments was 6.7, 13.3, +23 nm, or more. Some pictures showed that the thicker filaments consisted of super-coiling of two thinner filaments, for example two 6.7-nm filaments coiled up to give the 13.3-nm filaments. This suggests that the nucleoprotein complex of a circular double-stranded DNA and inner proteins of ad12 virus was folded in a stepwise fashion to produce the compacted form of the core.