Viroids and virusoids are the smallest subviral pathogens. Virusoids refer to a specific group of satellite RNAs, associated with sobemoviruses, that share common structural properties with viroids. Both pathogens are single-stranded, circular RNAs that do not encode any proteins. They replicate in infected host cells via RNA-RNA rolling circle mechanisms. Despite these similarities, they differ in their biological properties. Viroid RNAs replicate by utilizing the nuclear or chloroplast transcription machinery encoded by their host plants, whereas virusoids replicate in the cytoplasm by utilizing the transcription machinery encoded by their helper viruses. Furthermore, while viroids are not encapsidated, virusoids are encapsidated by the helper viral coat proteins. There is ample evidence that viroid RNAs contain all the information necessary to direct intracellular trafficking, replication, systemic trafficking, and pathogenicity. The biological functions of virusoids remain elusive.

Original languageEnglish
Title of host publicationEncyclopedia of Microbiology, Third Edition
Number of pages11
ISBN (Electronic)9780123739445
StatePublished - Jan 1 2009


  • RNA replication
  • RNA silencing
  • RNA trafficking
  • chloroplast
  • nucleus
  • phloem
  • plasmodesmata
  • small RNAs
  • viroid
  • virusoid


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