Viroids and Prions

Viroids and Prions - Viroids are a newly recognized unique class of pathogens which differ from viruses in the absence of a dormant phase (virions) and in the having much smaller genomes. They lack the protein component covering the nucleic acid of a virus, and consist of an RNA molecule only.

The first viroid discovered was the potato spindle tuber viroid (PSTV) I which causes a disease in potatoes. Diner and Raymer (1967) reported that the infectious agent for the disease was not a conventional virus but free RNA. PSTV is a circular RNA molecule whose nucleotide sequence and secondary structure has been established. It consists of only nucleotides, and does not contain the initiation codon (AUG) for protein synthesis.

Most of the nucleotides are base paired, producing a dsRNA molecule. This structure protects the viroid from the action of ribonucleases, which generally prefer ssRNA: The naked RNA molecule can multiply in plant cells. It can disperse, into the environment and produce diseases.

The molecular weight of PST V RNA is just sufficient to code for a very small protein of 70- 80 amino acids. This has led to the suggestion that PSTV is not a single RNA species, but a population of several small RN A molecules of the same size but with "different nucleotide sequences.

These molecules were together supposed to constitute a viral genome of a size comparable to that of a typical viral genome. Existing knowledge, however, does not support this model, and the current view is that the PSTV genome is very small.

Another suggestion was that PSTV may be associated with, a helper virus. However, it was experimentally found tbat when low molecular weight PSTV was simultaneously inoculated into different species of solanaceous plants, all showed signs of infection.

This makes the possibility of helper viruses being associated with PSTV unlikely, because this would require the assumption that helper viruses are universally present in apparently normal plants of different solanaceous species. PSTV appears to have the ability to replicate autonomously in susceptible plants in spite of its small size.