Pospiviroidae is an 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. The severity of the infection can vary from no effect to devastating and widespread damage to a population. This can also depend on the virus-host combination.
Members of the family Pospiviroidae have circular ssRNA of 246âÂÂ375 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 occurs in the nucleus and is mediated by the host enzyme DNA-dependent RNA polymerase II, which performs rolling circle replication using RNA templates. Positive-sense circular RNA molecules are transcribed into complementary negative-sense RNA strands containing multiple connected copies (a concatemer) of the genome. These negative-sense strands are used as templates to create concatemers of positive-sense strands that are cleaved into individual strands by a host RNAse III enzyme. The ends of the resulting linear molecules are connected by the host DNA ligase 1 enzyme to create genomic circular RNA.