CHAPTER 44 



teriophage,'' and many viruses attacking 

 plants (notably including tobacco mosaic 

 virus and turnip yellow mosaic virus). 



Before considering the genetics of RNA 

 viruses, it will be desirable to describe briefly 

 some features regarding their assay and life 

 cycle. Although these comments will pertain 

 specifically to the influenza virus, many of 

 them apply also to other animal and, to some 

 extent, some plant viruses. Because influenza 

 virus does not usually lyse the cell it infects, 

 it cannot be assayed, like phage, by its pro- 

 duction of plaques. In the case of this virus, 

 and others that produce no clear host-lethal 

 effect, the technique of limit dilution, to be 

 described, must be used for their detection. 

 This detection is facilitated, in the case of 

 influenza, by the use of a viral strain which is 

 capable of growing in the cells that line the 

 fluid-containing cavities of the chick embryo. 

 The virus produces a pathogenic eff'ect in 

 these cells. Accordingly, a sample to be 

 assayed for influenza virus is sufficiently di- 

 luted, and aliquots inoculated into a series of 

 eggs. Two or so days later, the eggs are 

 examined for pathogenic effect, to determine 

 the fraction which contain virus. If near- 

 limit dilutions are used, so that the proba- 

 bility is low that an aliquot contains a single 

 virus particle, one can estimate the virus 

 content of the entire sample. Under these 

 conditions, the progeny virus particles in a 

 given egg comprise a clone. 



The mammalian cell, which usually serves 

 as host for the influenza virus, typically 

 possesses a flexible shape and has an ambigu- 

 ous cell membrane surrounded by a mucoid 

 coat. This coat acts as a virus receptor since 

 it serves as a substrate for an enzyme located 

 on the virus surface. The virus has an outer 

 protein coat, containing the mucin-reacting 

 enzyme, and an inner core of RNA. (The 

 virus cannot attach to the cell if the mucoid 

 coat is removed by special treatment.) After 

 attachment, the particle enters the cell, per- 

 ^ See reference on p. 305. 



haps by being engulfed via the cell's normal 

 pseudopodial activity. Once inside the cell, 

 the particle enters an eclipse phase and multi- 

 plies vegetatively, during which period the 

 cell's RNA content increases. Some hours 

 after infection intact viral particles are hber- 

 ated gradually, not in large spaced clusters. 



FIGURE 44-3. Electron micrographs of tobacco 

 mosaic virus {TMV) slwwing its general configura- 

 tion {top) and its liollow core {middle) . Tiie bottom 

 photo shows a particle whose protein has been 

 partially removed by treatment with detergent, 

 leaving a thinner strand of RNA. {Courtesy of 

 R. G^ Hart.) 



