Viroid
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Pospiviroidae |
Viroids are plant pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary, circular, single-stranded RNA without the protein coat that is typical for viruses. The smallest so far is a 220 nucleobase scRNA (small cytoplasmic RNA) associated with the rice yellow mottle sobemovirus (RYMV)[1]. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2 kilobases in size.
A virus is a submicroscopic particle that can infect the cells of a biological organism. At the most basic level, viruses consist of genetic material contained within a protective protein shell, which distinguishes them from other virus-like particles such as prions (only protein) and viroids (nucleotides of RNA without protein coat).
Prion (IPA: [ˈpriːɒn]; 'prē,än[2]; "pree-on"[3]) is short for proteinaceous infectious particle that lacks nucleic acid (by analogy to virion, a single infectious viral particule) and is a type of infectious agent made only of protein. At the most basic level, the lack of nucleic acids distinguishes prions from viruses (genetic material contained within a protective protein shell) and viroids (nucleotides of RNA without protein coat).
Viroids were discovered and given this name by Theodor O. Diener, a plant pathologist at the Agricultural Research Service in Maryland, in 1971. [4] [5]
Viroid RNA 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 are ribozymes, having 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
Taxonomy
- Family Pospiviroidae
- Genus Pospiviroid; type species: Potato spindle tuber viroid
- Genus Hostuviroid; type species: Hop stunt viroid
- Genus Cocadviroid; type species: Coconut cadang-cadang viroid
- Genus Apscaviroid; type species: Apple scar skin viroid
- Genus Coleviroid; type species: Coleus blumei viroid 1
- Family Avsunviroidae
- Genus Avsunviroid; type species: Avocado sunblotch viroid
- Genus Pelamoviroid; type species: Peach latent mosaic viroid
Viroids and RNA silencing
There has long been confusion over how viroids are able to induce symptoms on plants without encoding any protein products within their sequences. Evidence now suggests that RNA silencing is involved in the process. Firstly, changes to the viroid genome can dramatically alter its virulence.[6] This reflects that fact that any siRNAs produced would have less complementary base pairing with target messenger RNA. Secondly, siRNAs corresponding to sequences from viroid genomes have been isolated from infected plants.[7] Finally, transgenic expression of the noninfectious hpRNA of potato spindle tuber viroid develop all the corresponding viroid like symptoms.[8]
This evidence indicates that when viroids replicate via a double stranded intermediate RNA, they are targeted by a dicer enzyme and cleaved into siRNAs that are then loaded onto the RNA-induced silencing complex. The viroid siRNAs actually contain sequences capable of complementary base pairing with the plant's own messenger RNAs and induction of degradation or inhibition of translation is what causes the classic viroid symptoms.
See also
ReferencesISBN links support NWE through referral fees
- ↑ Collins RF, Gellatly DL, Sehgal OP, Abouhaidar MG (1998). Self-cleaving circular RNA associated with rice yellow mottle virus is the smallest viroid-like RNA. Virology 241 (2): 269-75.
- ↑ Oxford University Press. 2002. The Oxford American College Dictionary. New York: G. P. Putnam's Sons.
- ↑ Pruisner, S. B. 1982. Novel proteinaceous infectious particles cause scrapie. Science 216(4542):136-44. Pronunciation is explicitly defined as "pree-on" on page 141, third column.
- ↑ Discovery of Viroids
- ↑ ARS Research Timeline - Tracking the Elusive Viroid (2006-03-02). Retrieved 2007-07-18.
- ↑ Elizabeth Dickson, Hugh D. Robertson, C. L. Niblett, R. K. Horst & Milton Zaitlin (1979). Minor differences between nucleotide sequences of mild and severe strains of potato spindle tuber viroid. Nature 277: 60-62.
- ↑ Papaefthimiou I, Hamilton A, Denti M, Baulcombe D, Tsagris M, Tabler M (2001). Replicating potato spindle tuber viroid RNA is accompanied by short RNA fragments that are characteristic of post-transcriptional gene silencing. Nucleic Acids Res. 29 (11): 2395-400.
- ↑ Wang MB, Bian XY, Wu LM, et al (2004). On the role of RNA silencing in the pathogenicity and evolution of viroids and viral satellites. Proc. Natl. Acad. Sci. U.S.A. 101 (9): 3275-80.
External links
Further reading
- WikiSpecies:Virus
- Wikipedia:WikiProject Viruses
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