Viruses: Bacterial, Animal, and Plant 



405 



The evidence suggests that the viral coat, 

 which contains some material made before 

 infection (by the host alone) and some made 

 after (by host and virus together), is added 

 around the RNA as this emerges from the 

 host cell. 



Two genetically different, haploid strains 

 of influenza virus are available, SWE (having 

 markers a c) and MEL (having markers A C). 

 It is possible to multiply infect a chick's egg 

 membranes with mixtures of the two strains. 

 Such mixed infections give progeny particles 

 which, when tested, yield pure clones, not 

 only of the parental genotypes, but also 

 of stable recombinant types (Ac or a C). 

 Since other explanations can be excluded, 

 the results prove that genetic recombina- 

 tion occurs also between RNA-containing vi- 

 ruses.^ 



No evidence has been obtained for the 

 occurrence of genetic recombination among 

 viruses attacking plants. In the case of the 

 tobacco mosaic virus {TMV), infection is 

 brought about experimentally by rubbing a 

 sample of virus on the leaf surface. Even 

 when a high concentration of virus is used, 

 only a small fraction of the virus particles 

 find and penetrate susceptible cells and give 

 rise to a demonstrable lesion. For this 

 reason, it is doubtful that one can multiply 

 infect a tobacco cell, so that experiments that 

 test for genetic recombination probably fail 

 to find it because of the lack of mixed infec- 

 tions. 



The TMV particle is a cylinder 3000 A long 

 and about 160 A in diameter (Figure 44-3 A). 

 It has a molecular weight of about 40 X 10^ 

 of which 38 X lO" is protein and 2 X 10« is 

 RNA.^ The outer dimensions of TMV are 



* Based upon the work of F. M. Burnet and others. 

 ' The RNA content of different small viruses com- 

 posed only of ribonucleoprotein is apparently con- 

 stant, also contributing about 2 X lO""' to the molecu- 

 lar weight of the virus particle. The protein contribu- 

 tion varies from 2 X lO^ to 100 X IQb, depending 

 upon the number of protein subunits present. 



due to the helical aggregation of about 2200 

 identical protein subunits; each subunit has 

 a molecular weight of about 18,000 and con- 

 tains 158 amino acids in a single polypeptide 

 chain (Figure 44-4). In cross section, the 

 TMV particle is seen to have a hollow core 

 about 40 A in diameter (Figure 44-3B), so the 

 protein subunit adds about 60 A to the radius. 

 The RNA in a particle (Figure 44-3C) is 

 typically a single, unbranched strand, consist- 

 ing of some 6400 nucleotides behaving as a 

 single molecule, which is threaded through 

 the protein subunits at a radius of 40 A. 

 Accordingly, the RNA is normally covered 

 externally by about 40 A 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 is treated with phenol, the 

 protein of the virus is destroyed, leaving the 

 single RNA molecule intact. When tobacco 

 is exposed to such RNA molecules, from 

 which protein is removed, some infection 

 occurs (the frequency is about 500 times less 

 than that obtained using an equal number of 

 whole virus particles), and typical TMV prog- 

 eny (complete with TMV protein coats) are 

 produced. Repeated phenol treatments do 

 not decrease the infectivity of the RNA and 

 no amount of protein can be detected chemi- 

 cally in these preparations. On the other 

 hand, RNAase destroys the infectivity of the 

 RNA fraction completely. It must be con- 

 cluded, therefore, that naked RNA is infective 

 and carries all the genetic information to repli- 

 cate itself}^ These experiments prove that 

 TMV protein plays no part in the replication 

 either of the RNA genetic material or of it- 

 self. This is further illustrated by what may be 

 called reconstitution experiments. It is pos- 

 sible, under certain conditions, first to sepa- 

 rate the protein and RNA of TMV, and then 

 to have them recombine to produce the high 



10 RNA isolated from a number of small animal 

 viruses is also infective. 



