Читать книгу Basic Virology - Martinez J. Hewlett - Страница 95

Knowledge of the particulars of a virus's structure and the basic features of its replication can be used in a number of ways to build a general classification of viruses. In 1971, David Baltimore suggested a scheme for virus classification based on the way in which a virus produces messenger RNA (mRNA) during infection. The logic of this consideration is that in order to replicate, all viruses must express mRNA for translation into protein, but how they do this is determined by the type of genome utilized by the virus. In this system, viruses with RNA genomes whose genome is the same sense as mRNA are called positive‐sense (+ sense) RNA viruses, while viruses whose genome is the opposite (complementary) sense of mRNA are called negative‐sense (− sense) RNA viruses. Viruses with double‐stranded genomes obviously have both senses of the nucleic acid.

The Baltimore classification has been used to varying degrees as a way of classifying viruses and is currently used mainly with reference to the RNA genome viruses, where positive‐ and negative‐sense viruses are grouped together in discussions of their gene expression features. This classification scheme is not complete, however. Retroviruses, which are positive sense but utilize DNA in their replication cycle, are not specifically classified. Still, the scheme provides a fundamental means of grouping a large number of viruses into a manageable classification.

A more general classification based on a combination of the Baltimore scheme and the three basic criteria listed above is shown in Table 5.2. When compared to the listing of viruses in Table 5.1, it is clear that this scheme is not complete; for example, viruses with complex morphology are not well represented. More importantly, subtle distinctions such as the actual genetic relatedness of the proteins involved in viral genome replication are not taken into account. Indeed, only those viruses that have been characterized in some detail, and whose infection has some medical or economic impact upon humans, have been included; if a virus is not a human pathogen or if its occurrence has no obvious economic impact, it has been ignored. While the scheme can be expanded to include all known viruses, it then loses the value of relative simplicity.