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Tobacco mosaic virus

The Tobacco mosaic virus (TMV) is an RNA virus that infects plants, especially tobacco, showing characteristic patterns (mottling and discoloration) on the leaves (thus the name). It was the first virus to be discovered.

In 1883 Adolf Mayer first described the disease that could be transferred between plants, similar to bacterial infections. However, in 1889, Martinus Beijerinck showed that a filtered, bacteria-free culture medium still contained the infectious agent. The first concrete evidence for its existence was given by Dmitri Ivanowski in 1892. In 1935, Wendell Meredith Stanley crystallized the virus for electron microscopy and showed that it remains active even after crystallization. For his work, he was awarded 1/4 of the Nobel Prize in Chemistry in 1946. In 1955, Heinz Fraenkel-Conrat and Robley Williams showed that purified TMV RNA and its capsid (coat) protein assemble by themselves to functional viruses, indicating that this is the most stable structure (the one with the lowest free energy), and likely the natural assembly mechanism within the host cell.

The crystalographer Rosalind Franklin worked for Stanley for about a month at Berkeley, and later designed and built a model of TMV for the 1958 World's Fair at Brussels. In 1958, she speculated that the virus was hollow, not solid, and hypothesized that the RNA of TMV is single-stranded. This conjecture was proven to be correct after her death.

Tobacco mosaic virus has a rod-like appearance. Its capsid is made from a single protein that assembles itself around the viral RNA in a helical structure (16.3 proteins per helix turn). The virions have a diameter ~18nm and an inner hole ~2nm. The protein coat consists of 158 aminoacids which are assembled into four main alpha-helices, which are joined by a prominent loop proximal to the axis of the virion. The RNA is located at a radius of ~4nm and is protected from the action of cellular enzymes by the coat protein. There are three RNA nucleotides per protein monomer.

The amino acid sequence of TMV's coat protein has been published in "American Chemical Society's Chemical Abstracts" in 1972 as a single word, Acetylseryltyrosylserylisol...serine, which supposedly is the longest word in the English language.

In plants, tobacco mosaic virus leads to severe crop losses. It is known to infect members of nine plant families, and at least 125 individual species, including tobacco, tomato, pepper, cucumbers, and a number of ornamental flowers. There are many different strains.

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