Viroids and Virusoids

Virusoid genomes are 220 to 388 nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate itself. Virusoids can replicate in the cytoplasm and possess a ribozyme activity. RNA replication is similar to that of viroids, but each requires that the cell be infected with a specific "helper" virus. Five virusoids are known, and the helper viruses for these are all members of the Sobemovirus family. An example of a "helper" virus is the subterranean clover mottle virus, which has an associated virusoid. Virus enzymes may aid replication of the virusoid RNA. The virusoid is incorporated into the virus particle and transmitted as a "satellite," a separate nucleic acid not part of the viral chromosome. Replication of the helper virus is independent of the virusoid.

Virusoids belong to a larger group of infectious agents called satellite RNAs, which are found in bacteria, plants, fungi, invertebrates, and vertebrates. Some satellite RNAs encode proteins; these are called satellite viruses and, like virusoids, must coinfect with a helper virus in order to replicate. One satellite RNA infecting humans is the hepatitis delta virusoid. It has a circular, single-stranded RNA genome of 1,700 nucleotides. Its helper is the hepatitis B virus, which is associated with liver disease. Coinfection with both agents results in a more severe infection.