VIRIODS AND PRIONS

PRESENTED BY-HASNAHANA CHETIAM.Sc Ist Sem.DEPTT. OF BIOTECHNOLOGY, G.U.

VIRIODS

Viriods are a group of virus-like pathogens much smaller and distinctly different from viruses, consists solely of small single-stranded circular RNA molecule and have no protein coat.

STRUCTURE

Viriods are covalently closed, circular, ssRNAs, about 250-370 nucleotides long. The circular RNA normally exist as a rod-like shape due to intra-strand base pairing, which forms double-stranded regions with single-stranded loops. The RNA of a viriod doesn’t encode any gene products;so they can’t replicate themselves. It is believed to adopt the host RNA Polymerase for this purpose which synthesizes a negative RNA strand. Then it serves as a template for the synthesis of the other viriod RNAs.

DISEASES CAUSED BY VIRIODS Viriods causes over 20 different

plant diseases of which the most studied are-

1. POTATO SPINDLE-TUBER DISEASE2. EXOCORTIS DISEASE OF CITRUS

TREES3. CHRYSANTHEMUM STUNT DISEASE

The only human disease caused by viriods is Hepatitis-D.

PATHOGENICITY

Though the pathogenicity of viriods is not well understood,it is presumed that viriods cause disease by triggering a eukaryotic response called RNA SILENCING, which normally functions to protect against infection caused by double stranded viruses, by degrading it. Viriods may usurp this response by hybridizing to specific host mRNA molecules to which they have a complementary sequence forming Hybrid viriod:host mRNA ds molecule which will elicit RNA silencing. This results in destruction of the host message and thus, silencing of the host gene. This leads to disease in the host plant.

POTATO SPINDLE TUBER VIRIOD

Potato Spindle Tuber Viroid (PSTV) is the first recognized viroid. PSTV is infectious RNA of low molecular weight, approximately 100,000 Daltons. The RNA is single stranded molecule of 359 nucleotides with extensive regions of base pairing. Under the electron microscope, purified but denatured PSTV appears as short strands about 50 nm long and thickness of double stranded DNA PSTVd is a small, circular RNA molecule. The natural hosts are potatoes (Solanum tuberosum) and tomatoes (Lycopersicon esculentum). All potatoes and tomatoes are susceptible to PSTVd and there is no form of natural resistance.

Electron microscopic picture of potato spindle tuber viroid (PSTVd)

PSTVd INFECTED POTATOES

HEPATITIS D

Hepatitis D is a disease caused by a viriod called Hepatitis delta virus(HDV). HDV is considered to be a subviral satellite because it can propagate only in the presence of HBV. The HDV genome exists as an enveloped negative sense, single-stranded, closed circular RNA. Because of a nucleotide sequence that is 70% self-complementary, the HDV genome forms a partially double stranded RNA structure that is described as rod-like.With a genome of approximately 1700 nucleotides, HDV is the smallest viriod known to infect animals.

PRIONS

Prions are infectious agents composed primarily of protein. They contain no nucleic acid. They cause a variety of neurodegenerative diseases in humans and animals.

PATHOGENICITY

The most studied prion is scrapie prion that causes the scrapie disease in sheeps. Its studies show that the causative agent,PrPSc, enters the brain of an animal converting the normal  PrPc  to PrPSc which continue this conversion chain by changing its folding patterns. Recent evidences suggest that  PrPSc  crosslinks PrPc ‘s to trigger apoptosis.

EC graph of Scrapie prion

Sheep infected with Scrapie prion

MODE OF TRANSMISSION

It has been recognized that prion diseases can arise in three different ways:

1. Acquired 2. Familial 3. Sporadic Current research suggests that the

primary method of infection in animals is through ingestion.

OTHER DISEASES CAUSED BY PRIONS In cattles- BOVINE SPONGIFORM

ENCEPHALOPATHY or Mad Cow disease in cows

In Humans-

Kuru Creutzfeldt-Jakob disease(CJD) Fatal Familial Insomnia Gerstmann-Strassler-Scheinker

syndrome

REFERENCES

MICROBIOLOGY BY PRESCOTT WIKIPEDIA SCIENCE DIRECT

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