1. Viroids
Dr. R. S. Jadhav
Department of Microbiology,
VNBN Mahavidyalaya, Shirala, Dist- Sangli.
2. Viroids
In 1967 Diener and Raymer first reported that the infectious agent for the potato spindle
tuber disease was not a conventional virus but a free RNA.
In 1971, Theodor Diener a plant pathologist discovered potato spindle tuber viroid. He
gave name the viroid. The first viroid discovered is the potato spindle tuber viroid (PSTV)
which causes a disease in potatoes.
Defination- The smallest known infectious agent consisting of a small circular RNA
molecule is called viroids.
Characteristics of viroids- 1) Viroids are obligate intracellular parasites.
2) Viroids are single stranded covalently closed circular RNA molecules.
3) they are infetious agents smaller than viruses.
4) The viroid RNAs are 246-400 nucletides long.
5) Viroid RNA does not code for any protein.
6) They do not have capsid (Protein coat).
Examples- Now days 33 viroids are reported. Most viroids cause plant diseases. The most
common viroids include Potato spindle tuber viroid, Citrus exocortis viroid, Cocconut
cadang-cadang viroid and Cucumber pale fruit viroid. Recently one has been reported to
cause a human diseases by a viroid is hepatitis D. This disease was previously ascribed to
a delta agent (defective virus). However, it is now known that the delta agent is a viroid
enclosed in a hepatitis B virus.
3. Structure
The viroids are ss circular RNA molecules. Most of the nucleotides in the RNA are base
paired, producing a ds RNA molecule. The ss RNA circle has extensive intrastrand base
pairing and unpaired loops at irregular intervals. This structure protects the viroid from the
action of ribonuclease. The central region of about 20 base pairs is highly conserved and is
called central conseverd region (CCR).
Viroids have five domains: Left terminal domain (TL), Pathogenicity domain (P), Central
conseverd region (CCR) Variable domain (V) and Right terminal domain (TR).
Replication-
Replication of viroids occurs in the nucleus. Viroid RNA is a plus strand RNA. Replication
of RNA occures by a rolling circle mechanism in vivo.
All components required for viroid- replication including the RNA polymerase are
provided by the host.
The rolling circle replication occurs by two mechanisms.
4. First Mechanisms- A RNA directed RNA polymerase catalyzes synthesis of new
concatemeric minus strand using the viroid plus RNA as template.
Viroids are ribozymes so catalyze self cleavege.nself cleavage of concatereric minus strand
to a monomer unit and ligation by host RNA ligase produce circular molecule. This circular
molecule is copied by the RNA polymerase to produce a concatameric plus strand. Cleavage
of a concatameric plus strand produces monomers which, on circularization produce viroids
(plus RNA).
5. In the sencond mechanism the concatemeric minus strand is copied directly
to a concatameric plus strand which is cleaved specifically to monomers.
The monomers are ligated to form many viroids.
The Hepatitis D viroid is a minus strand that gives rise to two RNA
molecules. One of these is a mRNA for the delta antigen and the other is a
complete plus strand RNA.
The plus strand acts as a template to make more minus strands. The minus
strand self cleaves and self ligates. The replication of HDV takes place in
the nucleus.
The delta antigen is made in the cytoplasm. Two molecules of delta antigen
bind to minus strand RNA as a dimer. The delta antigen is necessary for
viroid assembly but its exact role is unknown.
Significance:- Viroids cause many plant diseases which in turn cause
economic losses to agriculture. Many viroids are RNA catalysts
(ribozymes).
