364 



CHAPTER 2S 



brought about experimental!} by rubbing a 

 sample of virus on the leaf surface. Even 

 when a high concentration of virus is used, 

 onlj a small fraction of the virus particles 

 (one in 10'') find and penetrate susceptible 

 cells and give rise to a detectable lesion. For 



figure 28-1. Electron micrographs of to- 

 bacco mosaic virus (I Ml) showing its gen- 

 eral configuration (top) and its hollow core 

 {middle). The 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.) 



this reason, it is difficult to multiply-int'ect 

 a tobacco cell, and experiments testing for 

 genetic recombination are probably negative 

 due to the lack of mixed infections. 



The TMV particle is a cylinder 3000A 

 long and about 80A in radius (Figure 28-1, 

 top). It has a molecular weight of about 

 40 times 10 6 , of which 38 times 10 6 is pro- 

 tein and 2 times 10 ,; is RNA. The outer 

 dimensions of TMV are attributed to the 

 helical aggregation of about 2200 identical 

 protein subunits, each of which has a molec- 

 ular weight of about 18,000 and contains 

 158 amino acids in a single polypeptide 

 chain (Figure 28-2). A cross section of 

 the TMV particle shows a hollow core about 

 20A in radius (Figure 28-1, middle); the 

 protein subunit, therefore, adds about 60A 

 to the radius. The RNA in a particle (Fig- 

 ure 28-1, bottom) is typically a single, un- 

 branched strand consisting of some 6400 

 nucleotides threaded through the protein 

 subunits at a radius of 40A. Internally the 

 RNA is normally covered by about 20A and 

 externally by about 40A of protein subunit. 

 Since the protein subunits are arranged in a 

 gently-pitched helix (49 subunits per three 

 turns), the RNA forms a helix of the same 

 pitch. 



When TMV in water is treated with 

 phenol, the protein of the virus is extracted 

 into the phenol, leaving the single RNA 

 molecule intact in the water. If the tobacco 

 plant is exposed to RNA molecules with pro- 

 tein thus removed, the frequency of infection 

 is about 500 times less than the frequency 

 obtained with an equal number of whole 

 virus particles; typical TMV progeny (com- 

 plete with TMV protein coats) are produced. 

 Repeated phenol treatments do not further 

 decrease the infectivencss of the RNA. and 

 no amount of protein can be detected in the 

 preparations. RNasc, on the other hand, 

 completely destroys the infectivencss of the 

 RNA fraction but not the infectiveness of 

 the whole virus. It must be concluded, 



