Viriods are infectious agents smaller than viruses. For the first time T.O. Diener and W.B. Raymer (1967) discovered potato spindle tuber viriods (PSTV) which caused a disease in potatoes. They are different from viruses:

The pathogen exists in vivo as an encapsulated in the RNA,
Virion like particles are not detected in the infected tissues,
The infectious RNA is of low molecular weight,
Despite its small size, the infectious RNA replicates,
The infectious RNA consists of one molecular species only.

Viriods are supposed to be the primitive viruses and must have originated from cellular RNAs. In most of the healthy plants, RNA synthesis on RNA template must occur. Viriods would have originated from this RNA as they did not induce the biosynthetic machinery of their host from their own replication. Except tRNA and 5S RNA, several low molecular weight RNAs have been found to be associated with several virus infections such as tobacco leaves infected by TMV in E. coli infected by QB phage in oncogenic RNA viruses, etc. It is supposed that viriods would have been originated from virus induced low molecular weight RNAs which later on adapted as replicating infectious entities. Therefore, viriods provide the evidence that they are the degerated virus entities. It seems that nucleic acid codes for an enzyme replicase which is essential for its replications. There are two possibilities for genome replication, RNA dependent replication and DNA dependent replication. In first type of replication, It appears that RNA directed RNA polymerase are present to a limited extent in the normal cell of plant and E. coli which may synthesize the RNA molecules directed by the RNA. In second type the viriods are transcribed from a cellular DNA of the host cell complementary to viriod RNA. In the infected cell new DNA may be produced with the infecting viriod RNA which serves as template, so there is RNA directed DNA polymerase (reverse transcriptase).

Prions are different from both viruses and viriods. These are proteinaceous infectious particles. Prions are known as causal agents for neurological disorders such as bovine spongiform encephalopathy (mad cow disease) in cattle and Creutzfeldt Jakob disease (CJD), Gerstmann-Straussler-Scheinker (GSS) disease, kuru and fatal familiar insomnia in humans.

It has been found that the entire open reading frame (ORF) of all known mammalian and avian Prp (prion protein) gene eliminates the possibilities that Prp arises from RNA splicing. The two exons of the Syrian pamster Prp gene are separated by a 10 kb intron, exon-1 encodes a portion of the 5' untranslated leader sequence while exon-2 encodes the ORF and 3' untraslated region. The mouse and sheep Prp contain three exons. The promoter region contains multiple copies of G-C rich and are devoid of TATA box.

The prions were also transmissible at reduced efficiency to non-transgenic mice and hamster. These findings provide genetic evidence for homiphilic interactions between Prp in the inoculums and Prp synthesizes by the host.

The mechanism of spread of the disease is not fully understood. Some workers are of the opinion that disease is transmitted by the Prp alone. The infectious pathogen is Prp which has been chemically modified. According to the opinion of the other group the genetic information cannot be transmitted by the protein between the hosta. Proteins are known to carry the genetic information, so the infectious agent is a Virio, a tiny scarpie- specific nucleic acid coated with Prp protein. Others believe that prion diseases are caused by unknown viruses with usual properties.

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