MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 12. VIRUSES

Structure of Viral Particles

The Structure of A wide variety of Viruses has been examined using the Electron microscope. Tobacco mosaic virus, for example, is rod-shaped, measuring 300 nm in length and 15 nm in diameter. The virus consists of a single RNA molecule containing 6,000 NUCLEOTIDES (Fig. 12-2). Its capsid is composed of 2,000 identical protein molecules arranged in a helix (Fig. 12-3). The most common shape for viral capsids is the icosahedron, a polyhedron with 20 faces. An icosahedron is a geometric figure with cubic Symmetry, formed by A large number of identical subunits and possessing a maximum internal volume (geodesic domes are constructed on this exact principle). Icosahedral viruses include many agents responsible for human respiratory diseases (Fig. 12-4), poliomyelitis, fowl pox, herpes simplex, human papilloma, various types of cancers in mice and other animals, and numerous plant diseases such as apple mosaic (Fig. 12-5, A), tobacco ringspot, cucumber mosaic, and legume pod mottle. PLANT AND ANIMAL Rhabdoviruses are bullet-shaped and surrounded by an outer lipoprotein envelope (Fig. 12-5, B). Rod-shaped viruses with helical symmetry, similar to tobacco mosaic virus, are most characteristic of plants; they often resemble a loose helix and appear as long, flexible threads (Fig. 12-5, B).

Class="center">Fig. 12-2. Tobacco mosaic virus (TMV) particles as viewed under an electron microscope

Fig. 12-3. Schematic representation of a TMV particle. The RNA is located in the center of the virus, enclosed in a protein coat (capsid) made up of 2,200 identical protein molecules, each containing 158 amino acid residues. These protein molecules form irregularly shaped subunits. The RNA molecule lies within a groove formed by the narrowest ends of the subunits and is shown at the top as a dark strand

Fig. 12-4. A. Adenovirus, one of the many viruses that cause respiratory infections in humans. It has an icosahedral shape. Each of its twenty faces is an equilateral triangle made up of protein subunits, for a total of 252 subunits. B. A model of the adenovirus constructed from 252 tennis balls

Fig. 12-5. RNA-containing plant viruses. A. A mixture of apple mosaic virus and tobacco mosaic virus. The apple mosaic virus is icosahedral, whereas the tobacco mosaic virus is rigid and elongated. B. Rhabdovirus particles in a Cell of the chili pepper (Capsicum frutescens), displaying their typical bullet-like shape. The virus is located in the perinuclear space between the inner and outer membranes of the nuclear envelope. It passes through the inner nuclear membrane, a fragment of which subsequently envelops the Viral Particles. C. Particles of a long, flexible virus within a cell of the cactus Zygocactus truncatus

One of the common Bacteriophages, T4 (Figs. 12-1, 12-6), has a more complex structure than the viruses discussed above. Measuring 100 nm in length, the bacteriophage consists of five distinct parts: a hexagonal HEAD, a tail, a contractile tail sheath, a base plate, and tail fibers. A long DNA molecule is tightly coiled within the phage head. Like other viruses, phage T4 can synthesize many Proteins that are not part of the viral particles themselves. These proteins are used for METABOLISM/36.html">DNA Replication and other Functions. One of them, the enzyme Lysozyme, causes the lysis of the bacterial cell at the end of the infection cycle. Distinct clearings, known as plaques, appear on bacterial colonies where active cell lysis has been caused by bacteriophages.

Fig. 12-6. Electron micrograph (A) and model (B) of bacteriophage T4; phages of this group infect Escherichia coli Bacteria. The hollow tail core is surrounded by a contractile sheath powered by specialized fibers containing ATP. At the end of the tail lies a base plate, from which six long fibers radiate outward. These fibers attach to the Introduction/37.html">Bacterial Cell wall and pull the base plate toward it. The tail proteins then contract, driving the tail like a hypodermic syringe into the bacterium; the DNA molecule, which is 650 times longer than the head, is "injected" into The Cell, leaving the protein coat outside

Other large, complex viruses may consist of multiple DNA or RNA molecules and various proteins. Such viruses can contain up to 200 genes. However, the majority of viruses exhibit a simple cubic or helical Organization and contain only a few genes; for instance, tobacco mosaic virus has a total of just three genes.

Infectious Properties of viruses

The protein capsids of individual viral particles are frequently surrounded or even replaced by a lipid-rich envelope. Many viruses feature surface projections, or spikes, composed of Glycoproteins and Lipids. The properties of the molecules that form the viral capsid, envelope, or spikes determine their infectious characteristics. For example, a virus entering an animal cell first binds to a specific receptor located on the cell's Plasma Membrane. Specific receptors are unknown for plant viruses; instead, they enter through damaged surfaces.

The spread of viral infection can be illustrated using the Influenza virus as an example (Fig. 12-7). Specific glycoproteins forming the spikes on the viral envelope dictate its infectious properties, which can be altered through recombination and mutation. Immunity to viruses is based on the recognition of specific viral envelope proteins; consequently, a new strain arising from recombination or mutation can cause disease in a previously immunized population. The Properties of the surface glycoproteins of the influenza virus change constantly, and therefore, the infectious Properties of Individual strains of this virus change accordingly.

Fig. 12-7. Influenza virus. The spikes on the envelope of individual viral particles are clearly visible

The interval between influenza epidemics corresponds to the time required for a new viral strain to emerge and stabilize. Influenza is unique among infectious diseases in its capacity to cause periodic global epidemics that pose a serious threat to human life. In the winter of 1968–1969, 50 million cases of the Hong Kong flu were registered in the United States, resulting in 70,000 deaths. The devastating 1918–1919 influenza pandemic swept the globe in three successive waves and claimed 20 million lives. Other viral infections, including plant diseases, may spread depending on the structural features and variations of their capsids and envelopes, but they remain less thoroughly studied than influenza.