Helical viruses appear rod shaped in the electron microscope. The rod can be flexible or stiff. The best studied example of a simple helical virus is tobacco mosaic virus (TMV). The TMV virion is a rigid rod 18 nm in diameter and 300 nm long (Fig. 2.2B). It contains 2130 copies of a single capsid protein of 17.5 kDa. In the right-hand helix, each protein subunit has six nearest neighbors and each subunit occupies a position equivalent to every other capsid protein subunit in the resulting network (Fig. 2.2A), except for those subunits at the very ends of the helix. Each capsid molecule binds three nucleotides of RNA within a groove in the protein. The helix has a pitch of 23 A and there are 161/3 subunits per turn of the helix. The length of the TMV virion (300 nm) is determined by the size of the RNA (6.4 kb).
Many viruses are constructed with helical symmetry and often contain only one protein or a very few proteins. The popularity of the helix may be due in part to the fact that the length of the particle is not fixed and RNAs or DNAs of different sizes can be readily accommodated. Thus the genome size is not fixed, unlike that of icosahedral viruses.
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