Pospiviroidae

The Pospiviroidae are a incertae sedis family of ssRNA viroids with 5 genera and 39 species, including the first viroid to be discovered, PSTVd, which is part of genus Pospiviroid. Their secondary structure is key to their biological activity. The classification of this family is based on differences in the conserved central region sequence. Pospiviroidae replication occurs in an asymmetric fashion via host cell RNA polymerase, RNase, and RNA ligase. Its hosts are plants, specifically dicotyledons and some monocotyledons.

Genome
Members of the family Pospiviroidae have circular ssRNA of 246–375 nt. They assume rod-like or quasi-rod-like conformations containing a central conserved region (CCR) and a terminal conserved hairpin (TCH) or a terminal conserved region (TCR). The genome of viroids does not encode any proteins.

Replication
Its replication is nuclear and mediated by DNA-dependent RNA polymerase II, which is redirected to use RNA templates through an asymmetric RNA–RNA rolling-circle mechanism. (+) polarity circRNA molecules (by convention the most abundant strand in vivo) are repeatedly transcribed into oligomeric complementary (−) RNAs. Such intermediates serve as templates for generating oligomeric (+) RNAs that are cleaved by a host enzyme of the RNase III class. The termini of the resulting linear monomers are ligated by the host DNA ligase 1 to generate the mature circular viroid RNA.

Taxonomy
Apscaviroid


 * Apple dimple fruit viroid
 * Apple scar skin viroid
 * Apscaviroid aclsvd
 * Apscaviroid cvd-VII
 * Apscaviroid dvd
 * Apscaviroid glvd
 * Apscaviroid lvd
 * Apscaviroid plvd-I
 * Apscaviroid pvd
 * Apscaviroid pvd-2
 * Australian grapevine viroid
 * Citrus bent leaf viroid
 * Citrus dwarfing viroid
 * Citrus viroid V
 * Citrus viroid VI
 * Grapevine yellow speckle viroid 1
 * Grapevine yellow speckle viroid 2
 * Pear blister canker viroid

Cocadviroid


 * Citrus bark cracking viroid
 * Coconut cadang-cadang viroid
 * Coconut tinangaja viroid
 * Hop latent viroid

Coleviroid


 * Coleus blumei viroid 1
 * Coleus blumei viroid 2
 * Coleus blumei viroid 3
 * Coleviroid cbvd-5
 * Coleviroid cbvd-6

Hostuviroid


 * Dahlia latent viroid
 * Hop stunt viroid

Pospiviroid


 * Chrysanthemum stunt viroid
 * Citrus exocortis viroid
 * Columnea latent viroid
 * Iresine viroid 1
 * Pepper chat fruit viroid
 * Pospiviroid plvd
 * Potato spindle tuber viroid
 * Tomato apical stunt viroid
 * Tomato chlorotic dwarf viroid
 * Tomato planta macho viroid