The First Virus-Tobacco Mosaic Virus (TMV)
The Effect TMV has on the Tobacco leaf
History of TMV
Certainly one of the most interesting viruses to be discovered. This is mainly due to the fact that the Tobacco Mosaic Virus (TMV) was the first virus to ever be discovered. The discovery of TMV was a 49-year process. Many scientists contributed to this biological breakthrough. Namely; Adolf Mayer, Dimitri Ivanovsky, Martinus Beijerinck (coined the term ‘virus’), and Wendell Stanley. Adolf Mayer began the process in 1886 when he discovered that a tobacco mosaic disease was infecting tobacco and Solanaceae family plants. In 1935, Wendell Stanley crystallized TMV and thus determined the structure of the first virus ever. Over the years, scientists have researched and modified Stanley’s findings to approximate the most accurate structure of TMV.
3D Structure of TMV
Structure of TMV
TMV is a positive-sense single stranded RNA virus. It has a rod-like spherical appearance as a result of the coat proteins self-assembling. There are 16.3 coat proteins per helical turn which encapsulates the hairpin loop structure of the RNA. Additionally, there are 2130 coat proteins that collectively form the viral capsid. Each protein monomer contains 158 amino acid arranged into 4 alpha-helices which are joined by a prominent loop. The coat proteins assemble around virions. The virions are 300nm in length and 18nm in width, which ultimately dictate the dimensions of the TMV that can be seen by the human eye under a microscope. (Study.com, 2016) The RNA strand is situated within a hollow inner channel that has an approximate radius of 6nm. The RNA genome consists of 3 nucleotides per protein monomer. The genome is 6.3-6.5kb in size. The 3’ terminal has a tRNA like structure whereas the 5’ terminus contains a methylated nucleotide cap. There are 4 open reading frames (ORFs) which encode a replicase, RNA polymerase, a movement protein and a capsid protein. The RNA strand is protected from cellular enzymes through means of core proteins that surround it. TMV contains a lipid envelope wherein lies the viral proteins used to infect the different plant organisms. Lastly, the unique structure of TMV allows it to withstand up to 50 degrees Celsius making it a thermostable virus.
Viruses are often classified through means of characteristics and structural features that they do or do not possess. The main identification and classification methods are based on morphology, chemical composition and mode of replication. Morphology refers to size and shape, and other structural compositions. Chemical composition refers to the genomic structure and features. Morphology and chemical composition thus lead to differing modes of replication within host cells. Based on the above factors, scientists have classified TMV as follows:
Group: Group IV, based on its’ positive sense single-stranded RNA genome.
Family: Virgaviridae, based on its’ genomic composition as well as its’ rod shaped structure, alpha-like replication proteins, ability to infect plants, lack of envelope, virions with a central canal, helical symmetry and viral replication is cytoplasmic.
Genus: Tobamovirus, based on the same elements as that mentioned above in the family. However, viruses belonging to this genus have a unique ability to infect tobacco, potato, tomato, squash and other solanaceae family plants.
Species: Tobacco Mosaic Virus, based on its’ primary ability to cause a mosaic disease on tobacco plants.
TMV Replication Cycle in Host
The TMV enters the plant host cell through the plasmodesmata. It does this by a process known as ‘mechanical inoculation’. After its’ entry TMV begins to shed its’ coat proteins thereby releasing the viral RNA strand into the host cell. During uncoating, the MetHel:Pol gene is translated to form the subunits; MetHel and RNA Polymerase. At the 3’ RNA, the tRNAhis will prime an intermediate negative sense RNA to yield multiple viral mRNA’s. These mRNA’s then encode the movement protein, coat protein, and RNA polymerase. This implies that TMV has the ability to self-replicate its’ genome. The above proteins then spontaneously assemble around virions in a highly organized fashion. At this stage there is a defined helical structure that has formed. This helix grows further by the addition of host cell protomers to its’s ends. The RNA now travels through the hollow channel of the TMV and forms a loop at the growing protomer end. The final step being that RNA may now conform and fit into the helical capsid, thus completing the replication process. This newly formed TMV strain then penetrated another host cell and repeats the process. The mode of replication and action of TMV is usually localized. However, if there is a lack of inhibitors or resistance within the plant, TMV will enter the systemic system and infect the entire plant organism. The effects and symptoms of TMV infection of plants can be seen by gradual discoloration of leaf veins until it forms a mosaic or mottled pattern across the area of infection. The result is stunted plant growth.
Protein Products of TMV gene
Please feel free to leave any comments or questions below. I will gladly answer to the best of my ability.
And just to be clear this is not the virus found in cigarettes which is killing your lungs!That would be TAR. Don't worry I'm not judging I'm a smoker as well.
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