Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 136

Some small RNA and DNA genomes enter cells from virus particles as naked molecules of nucleic acid, whereas others are always associated with specialized nucleic acid-binding proteins or enzymes. A fundamental difference between the genomes of viruses and those of their hosts is that although viral genomes are often covered with proteins, they are usually not bound by histones in the virus particle (polyomaviral and papillomaviral genomes are exceptions). However, it is likely that all viral DNAs become coated with histones shortly after they enter the nucleus.

While viral genomes are all nucleic acids, they should not be thought of as one-dimensional structures. Virology textbooks (this one included) often draw genomes as straight, one-dimensional lines, but this notation is for illustrative purposes only; physical reality is certain to be dramatically different. Genomes have the potential to adopt amazing secondary and tertiary structures in which nucleotides may engage in long-distance interactions (Fig. 3.9).

The sequences and structures near the ends of viral genomes are often indispensable for replication. For example, the DNA sequences at the ends of parvovirus genomes form T-shaped structures that are required for priming during DNA synthesis. Proteins covalently attached to 5′ ends, inverted and tandem repeats, and bound tRNAs may also participate in the replication of RNA and DNA genomes. Secondary RNA structures may facilitate translation (the internal ribosome entry site [IRES] of picornavirus genomes) and genome packaging (the structured packaging signal of retroviral genomes, [Fig. 3.9]).

Figure 3.9 Genome structures in cartoons and in real life. (A) Linear representation of a picornavirus RNA genome. UTR, untranslated region. (B) Long-distance RNA-RNA interactions in a tombusvirus RNA genome. The 4,252-nucleotide viral genome is shown with secondary RNA structures at the 5′ and 3′ ends. Sequences that base-pair are shown in blue (required for RNA frameshifting) and red (required to bring ribosomes from the 3′ end to the 5′ end). Courtesy of Anne Simon, University of Maryland. (C) Schematic representation of RNA secondary-structure elements in the human immunodeficiency virus type 1 5′ leader, including the core packaging signal. (D) NMR structure of the RNA shown in C, without elements colored black. Courtesy of Paul Bieniasz, Rockefeller University.