Group IV Viruses: Single-Stranded (+)Sense RNA

The Group IV viruses have a positive sense genome. Positive sense RNA can be translated directly into protein, without a DNA intermediate and without creating a complementary RNA strand. To replicate its genome, though, a complementary DNA strand is required. The positive RNA strand serves as a template for an RNA-dependent RNA polymerase, yielding a complementary RNA strand, to form a dimer with the template strand. The double-stranded RNA subsequently serves as the template for a new positive sense genome. The positive strand RNA genome is independently infectious, for most Group IV viruses. This means that in the absence of a capsid, envelope, or enclosed proteins, the RNA molecule, when inserted into a cell, is capable of using host cell machinery to construct additional viruses. There are six subclasses of the Group IV single-stranded positive-sense RNA viruses: Picornaviridae, Togaviridae, Coronaviridae, Hepeviridae, Caliciviridae, Flaviviridae, and Astroviridae. As expected, within each class, viruses share structural similarities; but there are no properties, other than the defining property of a (+)-sense single stranded RNA genome, that extends to all six classes of the Group IV viruses. For example, some classes have envelopes (i.e., Flaviviridae, Togaviridae, Coronaviridae), and others do not (i.e., Caliciviridae, Picornaviridae, Hepeviridae, and Astroviridae). Keywords West nile fever, Yellow fever, Dengue fever, Epidemic polyarthritis, Sindbis fever, Western equine encephalomyelitis, German measles, Hepatitis