Viroid Satellite Prion
June 9‐11, 2010
A variety of novel infec7ous agents cause disease in plant, animal and human. Several types of nonviral, subcellular pathogens have disease –causing‐ poten7al. These include Satellite, Viroid and Prion Viroid, Satellite and Prion are unusual infec7ous agents characterized by having a very small genome and in the case of prions, possibly no genome at all. Because of their simpliﬁed structures satellite, prions and viroids are some7mes called subviral par7cles Conven7onal strategies to combat virus infec7ons, such as drug and vaccines, have no eﬀect on these unconven7onal agents. A beEer understanding of the biology of these novel infec7ous en77es will be necessary before means of treatment of the disease they cause will become available.
Viroids are common plant pathogens which are a serious economic problem but have recently been reported to cause a human disease. 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
Discovery of Viroids
Potato spindle tuber and at least 15 other crop diseases are caused by viroids, an en7ty that nobody had ever heard of before 1971, its oﬃcial date of discovery. TheodorO. Diener, the Agricultural Research Service plant pathologist who discovered the pathogen, named it the “viroid,”because it is “like a virus.” Like a virus, the viroid invades a cell and hijacks its reproduc7ve mechanisms. It forces the cell to duplicate the viroid's RNA instead of its own. The diﬀerence between viroids and RNA viruses is that viroids have no protec7ve protein coat. The scien7ﬁc dogma in 1971 was that an organism with no protein wasn't supposed to be able to replicate itself, even with a host cell's help. And an en7ty as small as the PSTV (potato spindle tuber viroid)—130,000 daltons—wasn't supposed to be able to infect anything, even a potato.
Potato spindle tuber
Potato spindle tuber Viroids are Circular, Single‐stranded, Non‐coding RNAs (~200‐400 nt.) that are able to infect certain plants. Under naGve condiGons the mature circular, (+)‐stranded viroid has a rod‐like secondary structure. This structure makes it parGally resistant against nucleases. (due to non‐encapsidated) ReplicaGon does not depend on the presence of a helper virus Because viroids do not code for any pepGde or protein, they have to u7lize proteins of the host for most biological funcGons like replicaGon, processing or transport. Three enzymaGc acGviGes are required for viroid replicaGon, an RNA polymerase, an RNAse and an RNA ligase.
Pospiviroidae VS Avsunviroidae
The structure of a Pospiviroid
Pospiviroid enclose CCR but lack of Ribozyme ac7vity Avsunviroid : lack a CCR but possess a ribozyme ac7vity Note. a ribozyme is a catalyGc RNA molecule, in this case RNA cleavage is the ribozyme acGvity
Addi7onally it is speculated that Avsunviroids may replicate in chloroplasts whereas Pospiviroids replicate in the nucleus and nucleolus.
Avsunviroids replicate via a Symmetric rolling circle mechanism, whereas Pospiviroids use an Asymmetric mechanism.
Symmetry and Asymmetry Replica7on the +ve infecGng circular RNA strand of a viroid serves as a template to make a large linear mulGmeric ‐ve strand.RNA pol II is probably the enzyme which does this. Pospiviroids with an asymmetric replica7on pathway then make +ve RNA from this long linear molecule. A host RNAse acGvity cleaves the +ve strand into unit viroid lengths. This molecule is ...