Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 188

The final mechanism for condensing the viral genome, by cellular proteins, is unique to polyomaviruses, such as simian virus 40, and papillomaviruses. The circular, double-stranded DNA genomes released from these virus particles are organized into nucleosomes that contain the four cellular core histones, H2A, H2B, H3, and H4, to form a minichromosome. Comparison of cryo-EM structures of purified particles of the human polyomavirus BK virus and virus-like particles formed only from VP1 (the major structural protein) has revealed two radial shells of the DNA genome within virus particles (Fig. 4.20B). The thickness of these shells (24 Å) and the distance between them match those of double-stranded DNA within a human nucleosome. The 20 or so nucleosomes that are associated with polyomaviral genomes condense the DNA by a factor of ~7. This packaging mechanism is elegant, with two major advantages: none of the limited viral genetic information needs to be devoted to DNA-binding proteins, and the viral genome, which is transcribed by cellular RNA polymerase II, enters the infected cell nucleus as a nucleoprotein closely resembling the cellular templates for this enzyme.