Viroid

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? Viroid Template:Taxobox begin placement virus
(unranked)	Subviral agents
(unranked)	Viroid

|- style="text-align:center; background:violet;" !Families |- | style="padding: 0 .5em;" | Pospiviroidae
Avsunviroidae |} Viroids are plant pathogens that consist of a short stretch (a few hundred bases) of highly complementary circular single-stranded RNA without the protein coat that is typical for viruses. The smallest so far is rice yellow mottle sobemovirus (RYMV), at 220 nucleotides. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves, the single-stranded circular DNA of circoviruses, is around 2 kilobases in size.

Notably, the RNA of viroids does not code for any known protein; some even lack the AUG initiation codon. The replication mechanism involves interaction with RNA polymerase II, an enzyme normally associated with synthesis of messenger RNA, and "rolling circle" synthesis of new RNA. Some viroids have "ribozyme", or RNA enzyme properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates. It has been proposed that viroids are "escaped introns".

Viroids are usually transmitted by seed or pollen. Infected plants can show distorted growth. The first viroid to be identified was the Potato spindle tuber viroid (PSTVd). Some 33 species have been identified.

Primary and secondary structure of the PSTVd viroid:

  1   CGGAACUAAA CUCGUGGUUC CUGUGGUUCA CACCUGACCU CCUGAGCAGA AAAGAAAAAA
 61   GAAGGCGGCU CGGAGGAGCG CUUCAGGGAU CCCCGGGGAA ACCUGGAGCG AACUGGCAAA
121   AAAGGACGGU GGGGAGUGCC CAGCGGCCGA CAGGAGUAAU UCCCGCCGAA ACAGGGUUUU
181   CACCCUUCCU UUCUUCGGGU GUCCUUCCUC GCGCCCGCAG GACCACCCCU CGCCCCCUUU
241   GCGCUGUCGC UUCGGCUACU ACCCGGUGGA AACAACUGAA GCUCCCGAGA ACCGCUUUUU
301   CUCUAUCUUA CUUGCUUCGG GGCGAGGGUG UUUAGCCCUU GGAACCGCAG UUGGUUCCU