Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 121

A universal function of viral genomes is to specify proteins. However, none of these genomes encode the complete machinery needed to carry out protein synthesis. Consequently, one important principle is that all viral genomes must be copied to produce messenger RNAs (mRNAs) that can be read by host ribosomes. Literally, all viruses are parasites of their host cells’ translation system.

A second principle is that there is unity in diversity: evolution has led to the formation of only seven major types of viral genome. The Baltimore classification system integrates these two principles to construct an elegant molecular algorithm for virologists (Fig. 3.1). When the bewildering array of viruses is classified by this system, we find seven pathways to mRNA. The value of the Baltimore system is that by knowing only the nature of the viral genome, one can deduce the basic steps that must take place to produce mRNA. Perhaps more pragmatically, the system simplifies comprehension of the extraordinary reproduction cycles of viruses.

The Baltimore system omits the second universal function of viral genomes, to serve as a template for synthesis of progeny genomes. Nevertheless, there is also a finite number of nucleic acid-copying strategies, each with unique primer, template, and termination requirements. We shall combine this principle with that embodied in the Baltimore system to define seven strategies based on mRNA synthesis and genome replication. The Baltimore system has stood the test of time: despite the discovery of multitudes of viral genome sequences, they all fall into one of the seven classes.

Replication and mRNA synthesis present no obvious challenges for most viruses with DNA genomes, as all cells use DNA-based mechanisms. In contrast, animal cells possess no known systems to copy viral RNA templates and to produce mRNA from them. For RNA viruses to propagate, their RNA genomes must, by definition, encode a nucleic acid polymerase.