INTRODUCTlON:
Five families of viruses are recognized as negative-stranded RNA viruses. These are the Arenaviridae, Bunyaviridae, Orthomyxoviridae, Paramyxoviridae, and Rhabdoviridae (Matthews, 1982). All have a lipid envelope, an external fringe of glycoprotein, and internal components consisting of one or more species of single-stranded RNA in addition to structural proteins and RNA polymerase components. Positive-stranded RNA viruses include members of the Picornaviridae, Caliciviridae, Togaviridae, Flaviviridae, Coronaviridae, and the non- enveloped insect and plant virus groups. The togaviruses, flaviviruses, and coronaviruses are enveloped and in that regard structurally resemble negative-stranded viruses except that they lack virion RNA polymerase activities. The picornaviruses, caliciviruses, and the insect and plant nonenveloped, positive-stranded viruses consist of single- strand viral RNA enclosed in structural proteins. Of the other major families of RNA viruses that have been recognized, the retroviruses are enveloped, diploid, single-stranded RNA viruses that have reverse transcriptase enzyme components within their virus particles; Reovlridae and Birnaviridae members are nonenveloped, multisegniented, double-stranded RNA viruses that have virion RNA polymerase components.
The feature that distinguishes the positive-stranded RNA viruses from the negative-stranded viruses is that their viral RNA functions as a messenger RNA (mRNA). For the picornaviruses, caliciviruses, and flaviviruses, the complete viral-size RNA is the only mRNA species; for at least some togaviruses and coronaviruses there are additional subgenomic, viral-sense RNA species (i.e., mRNA species that represent parts of the viral RNA sequence). Upon introduction into a cell, the viral RNA of positive-stranded viruses is translated by cellular ribosomes and the derived gene products initiate the processes that lead to a productive infection (i.e., replication of the RNA and synthesis of all the usual RNA, mRNA, protein, and virus forms). Due to these properties, the viral RNA of positive-stranded viruses can be infectious per se. The RNA species of negative-stranded viruses, retroviruses, and the double-stranded RNA viruses are not infectious per se since the infection processes of each of these viruses requires that the virion polymerase transcribes the viral RNA into complementary sequence mRNA species (reoviruses, birnaviruses, negative-stranded viruses), or DNA copies (retroviruses), before the infection can proceed. Therefore, removal of viral protein from the RNA of these viruses eliminates the required enzymes and renders the RNA noninfectious.
Two families of negative-stranded viruses have single species of genomic RNA (Rhabdoviridae, Paramyxoviridae); the others have seven or eight (Orthomyxoviridae), three (Bunyaviridae), or two species (Arenaviridae). In this chapter, the evidence will be reviewed that shows that arenaviruses and members of one genus of the Bunyaviridae (Phleboviruses) have some proteins coded in subgenomic, viral-sense mRNA species and other proteins coded in subgenomic, viral-complementary mRNA sequences (i.e., the viruses have genomes with an ambisense coding strategy). This unique feature is discussed in relation to the implications it has on the intracellular infection process and how such a coding arrangement may have evolved.