page_title: Virusoid
section_title: Replication
hierarchical_section_title: Virusoid / Replication
caption_attribution_description: Sticks bond diagram of full-length hammerhead ribozyme, as determined from the 2.2 Å crystal structure. The figure was created with PyMOL Stem I is in magenta and yellow, stem II is cyan, and stem III is red. The cleavage site nucleotide, C17, is shown in green, and the scissile phosphate is white.
context_page_description: Virusoids are circular single-stranded RNA dependent on viruses for replication and encapsidation. The genome of virusoids consist of several hundred nucleotides and does not code for any proteins.
  Virusoids are essentially viroids that have been encapsulated by a helper virus coat protein. They are thus similar to viroids in their means of replication and due to the lack of genes, but they differ in that viroids do not possess a protein coat. They encode a hammerhead ribozyme.
  Virusoids, while being studied in virology, are subviral particles rather than viruses. Since they depend on helper viruses, they are classified as satellites. Virusoids are listed in virological taxonomy as Satellites/Satellite nucleic acids/Subgroup 3: Circular satellite RNA.
context_section_description: The circular structure of virusoid RNA molecules is ideal for rolling circle replication, in which multiple copies of the genome are generated in an efficient manner from a single replication initiation event. Another advantage to circular RNAs as replication intermediates is that they are inaccessible and resistant to exonucleases. Additionally, their high GC content and high degree of self-complementarity make them very stable against endonucleases. Circular RNAs impose constraints on RNA folding by which secondary structures that are favored for replication differ from those assumed during ribozyme-mediated self-cleavage. 
  Plant satellite RNAs and virusoids depend on their respective helper viruses for replication, while the helper viruses themselves are dependent upon plants to provide some of the components required for replication. Therefore, a complex interaction involving all three major players including satellites, helper viruses and host plants is essential for satellite / virusoid replication.   
  satLTSV replication has been shown to occur through the symmetric rolling circle mechanism, wherein the satLTSV self-cleaves both (+) and (-) strands. Both the (+) and (-) strands of satLTSV were found to be equally infectious. Nevertheless, since only the (+) strand is packaged in the LTSV particles, the origin of assembly sequence (OAS) / secondary structure is assumed to be present on the (+) strand only. 
  Gellatly et al., 2011 demonstrated that the entire satLTSV molecule possesses sequence and structural significance wherein any mutations (insertions / deletions) causing disruption in the overall rod-like structure of the virusoid molecule are lethal to its infectivity. Foreign nucleotides introduced into the molecule will only be tolerated if they preserve the overall cruciform structure of the satLTSV. Furthermore, the introduced foreign sequences are eliminated in successive generations to ultimately reproduce the wild-type satLTSV. 
  Therefore, in satLTSV RNA, the entire sequence seems to be essential for replication. This contrasts with satRNA of TBSV or the defective-interfering RNAs, in which only a small portion of their respective sequences / secondary structures were found to be sufficient for replication.   
caption: A hammerhead ribozyme, not from a virusoid (PDB: 2GOZ​)
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