Featured Research

from universities, journals, and other organizations

Viroids: Molecular Vestiges Of The RNA World

Date:
May 17, 2009
Source:
Centro de Astrobiologia CSIC-INTA
Summary:
Viroids are unique systems for the study of RNA structure, function and evolution. They are the minimal RNA replicons characterized so far their genome is ten-fold smaller than that the smallest known vius RNA and they can therefore be considered in a certain sense at the frontier of life. Despite being only composed by a single-stranded circular RNA of 246-401 nt not encoding any protein, viroids contain sufficient information to infect some host plants, to manipulate their gene expression for producing a viroid progeny and, as a consequence, to incite in most cases specific diseases.

Viroids are unique systems for the study of RNA structure, function and evolution. They are the minimal RNA replicons characterized so far their genome is ten-fold smaller than that the smallest known vius RNA and they can therefore be considered in a certain sense at the frontier of life.

Related Articles


Despite being only composed by a single-stranded circular RNA of 246-401 nt not encoding any protein, viroids contain sufficient information to infect some host plants, to manipulate their gene expression for producing a viroid progeny and, as a consequence, to incite in most cases specific diseases. In contrast to viruses, which can be essentially regarded as parasites of the translation machinery of their host, viroids are parasites of their host transcription machinery.

The approximately 30 known viroids are grouped in the families Pospiviroidae, type species Potato spindle tuber viroid (PSTVd), and Avsunviroidae, type species Avocado sunblotch viroid (ASBVd). PSTVd and ASBVd replicate and accumulate in the nucleus and the chloroplast, respectively, and the same presumably occurs with the other members of both families. This classification scheme is supported by other criteria including the presence of hammerhead ribozymes in members of the family Avsunviroidae and of a central conserved region in members of the family Pospiviroidae.

As opposed to plant viruses, which encode proteins that mediate their own replication and movement, viroids depend exclusively on host factors for these purposes. Viroids replicate through an RNA-based rolling circle mechanism with three steps: i) synthesis of longer-than-unit strands catalyzed by a host nuclear or chloroplastic RNA polymerase that reiteratively transcribes the initial circular template, ii) processing to unit-length, which remarkably is mediated by hammerhead ribozymes in the family Avsunviroidae, and iii) and circularization resulting from the action of an RNA ligase or from self-ligation.

Among the many pending issues, how viroids redirect the template specificity of certain host DNA-dependent RNA polymerases to transcribe RNA, is one of the most challenging. In addition, viroids must recruit host factors for their intracelular, cell-to-cell and long-distance movement within the plant. There are also pending questions in this context, the most appealing of which is how members of the family Avsunviridae gain access into the chloroplast; because essentially no other RNA has been reported to traffic inside this organelle, the answer to this question may reveal novel transport pathways in plant cells.

Recently, viroid-specific RNAs with the characteristic properties of the small interfering RNAs, the most reliable markers for post-transcriptional gene silencing (PTGS), have been detected in plants infected by members of both families. This indicates that viroids are inducers and targets (and perhaps even suppressors) of PTGS, and that processes of this kind may mediate many aspects of viroid biology including pathogenesis.

Finally, certain viroid properties, prominent among which are their size, circularity and specially the presence of hammerhead ribozymes in members of the family Avsunviroidae, support the idea that viroids may have a very ancient evolutionary origin independent of viruses. Viroid origin would go back to the RNA world postulated to have preceded the present world on Earth based on DNA and proteins.


Story Source:

The above story is based on materials provided by Centro de Astrobiologia CSIC-INTA. Note: Materials may be edited for content and length.


Cite This Page:

Centro de Astrobiologia CSIC-INTA. "Viroids: Molecular Vestiges Of The RNA World." ScienceDaily. ScienceDaily, 17 May 2009. <www.sciencedaily.com/releases/2009/05/090511091731.htm>.
Centro de Astrobiologia CSIC-INTA. (2009, May 17). Viroids: Molecular Vestiges Of The RNA World. ScienceDaily. Retrieved October 31, 2014 from www.sciencedaily.com/releases/2009/05/090511091731.htm
Centro de Astrobiologia CSIC-INTA. "Viroids: Molecular Vestiges Of The RNA World." ScienceDaily. www.sciencedaily.com/releases/2009/05/090511091731.htm (accessed October 31, 2014).

Share This



More Plants & Animals News

Friday, October 31, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

How A Chorus Led Scientists To A New Frog Species

How A Chorus Led Scientists To A New Frog Species

Newsy (Oct. 30, 2014) A frog noticed by a conservationist on New York's Staten Island has been confirmed as a new species after extensive study and genetic testing. Video provided by Newsy
Powered by NewsLook.com
Surfer Accidentally Stands on Shark, Gets Bitten

Surfer Accidentally Stands on Shark, Gets Bitten

AP (Oct. 30, 2014) A 20-year-old competition surfer said on Thursday he accidentally stepped on a shark's head before it bit him off the Australian east coast. (Oct. 30) Video provided by AP
Powered by NewsLook.com
Ebola Inflicts Heavy Toll on Guinean Potato Trade

Ebola Inflicts Heavy Toll on Guinean Potato Trade

AFP (Oct. 30, 2014) The Ebola epidemic has seen Senegal and Guinea Bissau close its borders with Guinea and the economic consequences have started to be felt, especially in Fouta Djallon, where the renowned potato industry has been hit hard. Duration: 02:01 Video provided by AFP
Powered by NewsLook.com
Genetically Altered Glowing Flower on Display in Tokyo

Genetically Altered Glowing Flower on Display in Tokyo

Reuters - Innovations Video Online (Oct. 30, 2014) Just in time for Halloween, a glowing flower goes on display in Tokyo. Instead of sorcery and magic, its creators used science to genetically modify the flower, adding a naturally fluorescent plankton protein to its genetic mix. Ben Gruber reports. Video provided by Reuters
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

Strange & Offbeat Stories


Plants & Animals

Earth & Climate

Fossils & Ruins

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins