GENETIC CONTROL 25 



2. Genetic Material of Ribonucleoprotein Viruses 



Many viruses of plants and animals contain no DNA; they are made of 

 RNA enclosed in protein. Viruses certainly bring to the cell a set of instruc- 

 tions as how to assemble building blocks of proteins and nucleic acids into 

 virus RNA and protein. Extensive modifications of the protein moiety of 

 tobacco mosaic virus can be brought about by chemical treatment without 

 adverse effect on virus infectivity. Substitution of 70 per cent of the protein 

 NH2 groups by chlorobenzoyl groups (Miller and Stanley, 1942), addition 

 of leucyl residues (Fraenkel-Conrat, 1953), removal of the terminal 

 threonine of the protein by carboxypeptidase (Harris and Knight, 1952) 

 have no effect on infectivity, nor on the nature of the virus produced. A 

 large part of the protein can be removed without killing the virus (Schramm 

 etal, \955). 



On the other hand, slight modifications to the virus RNA result in virus 

 inactivation. Structural analogues of purines and pyrimidines (e.g. 8- 

 azaguanine, 2-thiouracil or 5-fluorouracil) drastically reduce multiplication 

 (Commoner and Mercer, 1951, 1952; Matthews, 1952; Gordon and 

 Staehelin, 1959); this must be due to alterations of the virus RNA by the 

 abnormal purines or pyrimidines, for a close correlation was found between 

 incorporation of thiouracil into the virus RNA and the inhibition of virus 

 multiplication (Jeener and Rosseels, 1953). If ribonuclease is introduced 

 into the cells of a tobacco plant at the beginning of infection, virus multi- 

 plication is blocked (Hamers-Casterman and Jeener, 1957; Benda, 1958). 

 Multiplication of influenza virus is also inhibited by ribonuclease (Le 

 Clerc, 1957). 



Such experimental data indicated that the integrity of RNA is more 

 essential to virus multiplication than integrity of the protein sheath (Jeener, 

 1956). Fraenkel-Conrat and Williams (1955) were able to separate the RNA 

 from the protein and to reassociate them into virulent particles. Virus 

 reconstituted by mixing RNA and protein from different strains always 

 caused lesions which resembled very much those corresponding to the 

 virus from which the RNA had been obtained (Fraenkel-Conrat, 1956; 

 Fraenkel-Conrat et al, 1957). Numerous mutants were formed, however, 

 as if the genetic material of the virus had been slightly damaged during the 

 process (Commoner et al, 1956; Fraenkel-Conrat and Singer, 1957; 

 Bawden and Pirie, 1957). Ribonuclease does not kill the intact virus, but 

 the slightest action of the enzyme upon the naked RNA leads to complete 

 inactivation of the reconstituted nucleoprotein (Fraenkel-Conrat and 

 Williams, 1955). 



Finally, Gierer and Schramm (1956) were able to isolate from tobacco 

 mosaic virus, by phenol extraction, RNA which could transmit infection. 

 This fundamental observation has now been confirmed by work from many 

 laboratories and extended to other viruses. Control experiments of various 



