Wednesday, 23 June 2021

Detailed Structure of Tobacco Mosaic Virus (TMV)

 TMV (Tobacco Mosaic virus) is a plant virus which infects a wide range of plants. TMV causes disease of Tobacco, Tomato and other members of the family Solanaceae. TMV belongs to Tobamovirus group.

TMV is first virus ever discovered by Iwanowski in 1892, and latter isolated by Stanley in 1935.

The virus infection causes characteristic patterns, such as
"Mosaic"-like mottling(spots) and discoloration on
the leaves (hence the name).

TMV has historical significance as it is

  • a first discovered
  • first crystallized
  • first sequenced plant virus to be shown to contain RNA as genetic material and
  • first virus to be shown to contain RNA and protein.

Symptoms of TMV on plant

  - Mosaic patterns
  - Mottling,
  - Necrosis,
  - Stunting,
  - Leaf curling
  - Yellowing of plant tissue.

Structure of TMV

• TMV is the best studied virus with helical naked capsid

• It is rod shaped virus of 300 nm length and 18 nm diameter. 

• The central opening along the axis has diameter of 4 nm. 

• The capsids are made up of subunits that are called capsomers.

• Capsid consists of 2130 identical capsomers. 

• Each capsomers is made up of 158 amino acids. 

• Capsomers are arranged in a helix around central hole of about 4nm radius. 

Structure of TMV virus

• TMV consist of a single stranded, unsegmented and positive sense RNA as a genetic material. 

• There are about 6395 nucleotide in RNA. 

• RNA has MW of 2.1 x 10⁶ daltans. 

• The RNA is arranged in helix.
• Each turn of RNA helix contains 49 nucleotides and 16.3 capsomeres are attached RNA per turn of helix. That means 3 nucleotide are linked with single capsomer.

• The ratio of nucleoids to capsomers is 3:1

Genome of TMV

The TMV genome contains 4 genes which are -
1]. MTH gene, (Methyl transferase & helicase gene)
  2]. RNP gene,
  3]. MP gene and
  4]. CP gene.

Genome of TMV virus
 
• The MTH gene codes for the enzyme methyl transferase and RNA helicase. The methyl transferase activity plays role in RNA capping and the role of RNA helicase is not clear.

• RNP gene codes for RNA dependent RNA polymerase or RNA replicase. It is useful for making the copies of RNA.

• MP gene codes for movement protein and this proteins mediate the cell to cell movement of the viral RNA in the plant tissues.

• CP gene codes for coat protein or Capsid proteins.

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