|Tobacco mosaic virus|
|Electron micrograph of TMV particles stained to enhance visibility at 160,000x magnification|
|Group:||Group IV ((+)ssRNA)|
|Species:||Tobacco mosaic virus
Tobacco mosaic virus (TMV) is an RNA virus that infects plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns (mottling and discoloration) on the leaves (hence the name). TMV was the first virus to be discovered. Although it was known from the late 19th century that an infectious disease was damaging tobacco crops, it was not until 1930 that the infectious agent was determined to be a virus.
In 1883, Adolf Mayer first described the disease that could be transferred between plants, similar to bacterial infections. Dimitri Ivanovski gave the first concrete evidence for the existence of a non-bacterial infectious agent, showing that infected sap remained infectious even after filtering through Chamberland filter candles, in 1892. However, he remained convinced despite repeated failures to produce evidence, that bacteria were the infectious agents. In 1898, Martinus Beijerinck showed that a filtered, bacteria-free culture medium still contained the infectious agent. Wendell Meredith Stanley crystallized the virus in 1935 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, even though it was later shown some of his conclusions (in particular, that the crystals were pure protein, and assembled by autocatalysis) were incorrect. The first electron microscopical images of TMV were made in 1939 by Gustav Kausche, Edgar Pfankuch and Helmut Ruska - the brother of Nobel Prize winner Ernst Ruska. 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 crystallographer 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 and is now known to be the + strand.
|Tobacco mosaic virus coat protein|
|A monomeric unit of the tobacco mosaic virus coat protein.|
Tobacco mosaic virus has a rod-like appearance. Its capsid is made from 2130 molecules of coat protein (see image to the left) and one molecule of genomic RNA 6400 bases long. The coat protein self-assembles into the rod like helical structure (16.3 proteins per helix turn) around the RNA which forms a hairpin loop structure (see the electron micrograph above). The protein monomer consists of 158 amino acids which are assembled into four main alpha-helices, which are joined by a prominent loop proximal to the axis of the virion. Virions are ~300 nm in length and ~18 nm in diameter. Negatively stained electron microphotographs show a distinct inner channel of ~4 nm. The RNA is located at a radius of ~6 nm and is protected from the action of cellular enzymes by the coat protein. There are three RNA nucleotides per protein monomer. X-ray fiber diffraction structure of the intact virus based on an electron density map at 3.6 Å resolution.
TMV is a thermostable virus. On a dried leaf, it can withstand up to 120 degrees Fahrenheit (50 °C) for 30 minutes.
TMV has an index of refraction of about 1.57.
Following entry into its host via mechanical inoculation, the TMV RNA genome is not immediately translated. Instead the RNA is processed by a mechanism that is not yet understood. The resulting mRNAs encode several proteins, including the coat protein and an RNA-dependent RNA polymerase (RdRp). Thus TMV can replicate its own genome. After the coat protein and RNA genome of TMV have been synthesized, they spontaneously assemble into complete TMV virions in a highly organized process. The protomers come together to form disks composed of two layers of protomers arranged in a helical spiral. The helical capsid grows by the addition of protomers to the end of the rod. As the rod lengthens, the RNA passes through a channel in its center and forms a loop at the growing end. In this way the RNA can easily fit as a spiral into the interior of the helical capsid.
When TMV infects a tobacco plant, the virus enters mechanically (for example, through a ruptured plant cell wall) and replicates. After its multiplication, it enters the neighboring cells through plasmodesmata. For its smooth entry, TMV produces a 30 kDa movement protein called P30 which tends to enlarge the plasmodesmata. TMV most likely moves from cell-to-cell as a complex of the RNA, P30, and replicase proteins. The first symptom of this virus disease is a light green coloration between the veins of young leaves. This is followed quickly by the development of a "mosaic" or mottled pattern of light and dark green areas in the leaves. These symptoms develop quickly and are more pronounced on younger leaves. Mosaic does not result in plant death, but if infection occurs early in the season, plants are stunted. Lower leaves are subjected to "mosaic burn" especially during periods of hot and dry weather. In these cases, large dead areas develop in the leaves. This constitutes one of the most destructive phases of tobacco mosaic virus infection. Infected leaves may be crinkled, puckered, or elongated.
Consumption of tobacco products infected with the tobacco mosaic virus has been found to have no effect on humans.
The tobacco mosaic virus has been known to cause a production loss for flue cured tobacco of up to two percent in North Carolina. It is known to infect members of nine plant families, and at least 125 individual species, including tobacco, tomato, pepper (all members of the useful Solanaceae), cucumbers, and a number of ornamental flowers. There are many different strains.
The large amount of literature about TMV and its choice for many pioneering investigations in structural biology (including X-ray diffraction), virus assembly and disassembly, and so on, are fundamentally due to the large quantities that can be obtained, plus the fact that it does not infect animals. After growing a few infected tobacco plants in a greenhouse and a few simple laboratory procedures, a scientist can easily produce several grams of virus. As a result of this, TMV can be treated almost as an organic chemical, rather than an infective agent.