Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 185

A definitive property of a virion is the presence of a nucleic acid genome. Incorporation of the genome requires its discrimination from a large population of cellular nucleic acid. This packaging process is described in Chapter 13. The volumes of closed capsids are finite. Consequently, accommodation of viral genomes necessitates a high degree of condensation and compaction. A simple analogy illustrates vividly the scale of this problem; packing of the ~150-kbp DNA genome of herpes simplex virus type 1 into the viral capsid is equivalent to stuffing some 10 ft of 22 American gauge wire (diameter, 0.644 mm) into a tennis ball. Such confinement of the genome can result in high internal pressure, equivalent to that generated in locomotive steam engines, some 18 and 25 atm within herpes simplex virus type 1 and phage capsids, respectively. Such pressure provides the force that powers ejection of DNA genomes. Packaging of nucleic acids is an intrinsically unfavorable process because of the highly constrained conformation imposed on the genome. In some cases, the force required to achieve packaging is provided, at least in part, by specialized viral proteins that harness the energy released by hydrolysis of ATP to drive the insertion of DNA. In many others, the binding of viral RNA or DNA genomes to capsid proteins appears to provide sufficient energy. The latter interactions also help to neutralize the negative charge of the sugar-phosphate backbone, a prerequisite for close juxtaposition of genome sequences.

We possess relatively little information about the organization of genomes within viral particles: nucleic acids or protein-nucleic acid assemblies are not visible in the majority of high-resolution structural studies reported. This limitation indicates that the genomes or internal structures lack the symmetry of the capsid, do not adopt the same conformation in every viral particle, or both. Nevertheless, three mechanisms for condensing and organizing nucleic acid molecules within capsids can be distinguished and are described in the following sections.