Introduction to RNA Viruses

RNA viruses replicate their genomes using virally encoded RNA-dependent RNA polymerase (RdRp). The RNA genome is the template for synthesis of additional RNA strands. During replication of RNA viruses, there are at least three types of RNA that must be synthesized: the genome, a copy of the genome (copy genome), and mRNAs. Some RNA viruses also synthesize copies of subgenomic mRNAs. RdRp is the key player for all of these processes. RdRps of all RNA viruses probably arose from a common ancestor. The RdRp and other proteins required for viral genome synthesis are often called the replicase complex. The replicase complex consists of the set of proteins required to produce infectious genomes. In addition to the RdRp, the replicase complex may contain RNA-helicases (to unwind highly base-paired regions of the RNA genome) and NTPases (to supply energy for the polymerization process). The number of proteins in the replicase complex differs among virus families. There may also be a requirement for host cell proteins. The RNA virus group can be subdivided based on the type of RNA that serves as the genome. Positive or plus (+)-strand RNA viruses have genomes that are functional mRNAs. Their genomes are translated shortly after penetration into the host cell to produce the RdRp (and other viral proteins) required for synthesis of additional viral RNAs. Positive-strand RNA viruses often use large complexes of cellular membranes for genome replication. They actively modify host cell membranes to construct viral replication scaffolds. There are three groups of RNA viruses whose genomes are not mRNAs. They are the negative- or minus-strand RNA viruses, the closely related ambisense RNA viruses, and double-stranded RNA viruses. For each of these groups of viruses, the first synthetic event after genome penetration is transcription. This is accomplished by viral proteins (including the RdRp) that enter cell with the genome.