Viroids
| Viroid | |
|---|---|
 |
|
| Scientific classification | |
| (unranked): | Subviral agents |
| (unranked): | Viroid |
| Families | |
Viroids are the smallest infectious pathogens known. They are solely composed of a short strand of circular, single-stranded RNA without protein coat. All known viroids are inhabitants of higher plants, in which most cause diseases, some of which are of slight to catastrophic economic importance.
The first recognized viroid, the pathogenic agent of the potato spindle tuber disease, was discovered, initially molecularly characterized, and named by Theodor Otto Diener, plant pathologist at the U.S Department of Agriculture's Research Center in Beltsville, Maryland, in 1971. This viroid is now called Potato spindle tuber viroid, abbreviated PSTVd.
Discovery of the viroid triggered the third major extension of the biosphere in history to include smaller lifelike entities—-after the discovery of the “subvisible” microorganisms by Antonie van Leeuwenhoek in 1675 and the “submicroscopic” viruses by Dmitri Iosifovich Ivanovsky in 1892. The unique properties of viroids have been recognized by the International Committee for Virus Taxonomy with the creation of a new order of subviral agents.
As cogently expressed by Flores et al.:“viruses (and viroids) share the most characteristic property of living beings: In an appropriate environment, they are able to generate copies of themselves, in other words, they are endowed with autonomous replication (and evolution). It is in this framework where viroids represent the frontier of life (246 to 467nt), an aspect that should attract the attention of anybody interested in biology.”.
Although viroids are composed of nucleic acid, they do not code for any protein. The viroid's replication mechanism uses RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid's RNA as a template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.
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