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Taxonomic ClassificationGenomic CharacteristicsReplication MechanismBiological and Agricultural Significance

Taxonomic Classification

Pospiviroidae is a family of incertae sedis viroids, composed of single-stranded RNA (ssRNA) molecules. The family includes:

  • 5 genera and 39 species, including the first viroid ever discovered, the Potato spindle tuber viroid (PSTVd), classified under the genus Pospiviroid.

The classification within this family is determined by variations in the conserved central region (CCR) of the viroid genome.

Hosts and Impact

  • The hosts are primarily dicotyledonous plants and, to a lesser extent, monocotyledons.
  • Infection severity ranges from asymptomatic to causing severe, widespread damage, depending on the viroid-host interaction.

Genomic Characteristics

Genome Structure:

  • Circular ssRNA genome of 246–375 nucleotides (nt) in length.
  • The genome forms rod-like or quasi-rod-like secondary structures, featuring a:
    • Central conserved region (CCR) critical for biological activity.
    • Terminal conserved hairpin (TCH) or terminal conserved region (TCR).

Lack of Protein Encoding:

  • The genome does not encode any proteins, relying entirely on host cellular machinery for replication and pathogenesis.

Replication Mechanism

Replication Site:

  • Nuclear replication mediated by DNA-dependent RNA polymerase II repurposed to use RNA templates.

Rolling-Circle Mechanism:

  • Replication proceeds asymmetrically via an RNA–RNA rolling-circle mechanism:
    • (+)-polarity circular RNA, the predominant strand in vivo, is transcribed into oligomeric complementary (-) RNAs.
    • These (-) RNA intermediates serve as templates to synthesize oligomeric (+) RNAs.
    • Host RNase III-class enzymes cleave the oligomeric (+) RNA into linear monomers.
    • Linear monomers are circularized by host DNA ligase 1, completing the replication cycle.

Biological and Agricultural Significance

The structural and replication dynamics of Pospiviroidae make them highly adaptive pathogens with the potential to cause devastating agricultural losses. Their unique RNA-only genome and replication strategy provide insights into host-pathogen interactions and RNA biology.