Viroids : Characteristics, Structure, Types and Replication

 Viroids are the smallest known infectious agents consisting of a small, circular, RNA molecules.
A VIROIDs are a Virus(VIR) like(OID) particles.
Until 1970, viruses were considered as the smallest infectious agents. However, the discovery of viroids has proved that the infectious entities smaller than virus exist in nature.

Diener & Raymer first time discovered the Potato Spindle Tuber Viroid (PSTV). It was responsible for potato spindle tuber disease.

  Most viroid cause plant discases & most common example is potato spindle tuber viroid. Today, 33 viroids are known.

The human disease known to be caused by viroid is hepatitis D. This viroid is enclosed in a hepatitis B virus capsid. For hepatitis D to occur, there must be simultaneous infection of cell with both the hepatitis B virus & the hepatitis D viroid.

Characteristics of Viroids :

  1. Viroids are obligate intracellular parasite.
  2. They are smaller than viruses. 
  3. Viroids are single stranded covalently closed circular RNA molecules.
  4. They are only 264-400 nucleotides long.
  5. Viroid RNA does not code for any protein.
  6. They use host polymerase for replication.
  7. They do not have capsid (protein coat).

Examples of Viroids

  • Genus Pospiviroids: PSTV (potato spindle tuber viroid)
  • Genus Coleviroids: CbVd 1 (coleus blumei 1)
  • Genus Hostuviroids: HSV (hop stunt viroid)
  • Genus Avsunviroids: ASBV (avocado sunblotch viroid)
  • Genus Cocadviroids CCCV (coconut cadang-cadang viroid)
  • Genus Pelamoviroids:  PLMVD (peach latent mosaic viroid)
  • Genus Apscaviroids: ASSVd (apple scar skin viroid)

Structure of Viroid

  • The viroids are single stranded circular RNA molecules.
  • Most of the nucleotides in the RNA are base paired, producing a double stranded RNA molecules.
  • The single stranded RNA circle has extensive intra-strand base pairing & unpaired loops at intervals.
  • This structure protects the viroid from the action of ribonuclease.
  • Structure of a viroid-circular single-stranded RNA with some pairing between complementary bases and loops where no such pairing occurs.
  • There are two main groups of viroids on the basis of structure, these are self cleaving and non- self cleaving.
  • Non-self cleaving viroids replicate in nucleus and fold into "dog bone" or rod-like structure.

Structure of Viroid 

  • Five domains identifiable in Non-self cleaving viroids
  - Terminal left (TL)
  - Terminal right (TR)
  - Pathogenicity (P)
  - Central (C)
  - Variable (V)

Replication of Viroids

  • The replication of the viroid takes place in the nucleus of the host cell.
  • Viroid RNA is a positive strand RNA and it replicates by the rolling circle mechanism in vivo.
  • All components required for the replication are provided by the host.
  • The rolling circle replication occurs by two mechanisms which are symmetric mode of replication and asymmetric mode of replication.

Symmetric mode of replication

  • Symmetric mode of replication is the most common mode of replication in viroids.
  • According to Symmetric mode of replication, RNA directed RNA polymerase catalyses synthesis of new concatemeric negative(—) strand using the viroid position (+) RNA as a template.
  • Viroids are ribozymes and therefore they catalyze their self cleavage.
  • Self cleavage of concatemeric negative strand produce a monomeric subunits
  • This is followed by the Ligation by host RNA ligase enzyme produce circular molecule.
  • This circular negative strand is copied by the RNA polymerase to produce a concatemeric positive strand.
  • Cleavage of concatemeric positive strand produces monomers.
  • These monomers again circularize and produce positive RNA i.e. viroids.

Replication of Viroids

Asymmetric mode of replication

  • In the Symmetric mode of replication, the concatemeric negative strand is copied directly to a concatemeric positive strand.
  • That means, Here self cleavage of monomers of negative strand is not carried out. Instead of that the intake of negative strand is used to synthesise the complementary concatemeric positive strand.
  • It is then cleaved specifically to monomers.
  • These monomers are ligated to form many viroids.