RNA IN PLANT VIRUSES 27 



cation, rods having a molecular weight of 100,000 are reconstituted. 

 But the interesting point is that an empty hole is present in the 

 protein units or aggregates. It looks as if the RNA, which dissolves 

 in sodium hydroxide, occupies the center of the virus particle. The 

 correctness of this view is shown in the beautiful electron micros- 

 cope photographs of Hart (1955) and Fraenkel-Conrat and Wil- 

 liams (1955). If the virus is treated with a detergent, filaments can be 

 seen to project from the remains of the virus particles ; these fila- 

 ments disappear after treatment with ribonuclease. The protein 

 has the structure of a pearl, in which a hole is bored for the filamen- 

 tous RNA. There is no doubt that RNA, which forms a single 

 strand, occupies the center of the particle and that it is surrounded 

 by a protein shell. 



The fact that the protein part of the virus is non-infectious, in 

 contrast to the whole virus, suggests that RNA is really essential for 

 the synthesis of plant viruses. If so, one could expect an inhibition 

 of virus multiplication on the addition of substances which interfere 

 with RNA synthesis. That this is the case has been shown con- 

 clusively by Commoner and Mercer (1952), who obtained complete 

 inhibition of synthesis of tobacco mosaic virus by thiouracil at a 

 concentration of 4.3 • 10~^ M. This inhibition was partially reversed 

 when uracil was added in concentrations of the same order of 

 magnitude. 



These findings of Commoner and Mercer (1952) have been con- 

 firmed by Jeener and Rosseels (1953), who obtained in addition 

 some quite unexpected results. They found that the inhibition of 

 virus synthesis is greater, the smaller the amount of virus present 

 in the leaves to which thiouracil is added. This observation cannot 

 be explained on the basis of a competition between thiouracil and 

 uracil in some enzymatic reaction during the synthesis of RNA. 

 It tends rather to indicate that thiouracil can be incorporated into 

 the virus RNA and that this incorporation hinders the further 

 multiplication of the modified particles. This interpretation of the 

 facts has been confirmed by experiments in which ^sS-labeled 

 thiouracil was added to leaves infected 2 days earlier. The concen- 

 tration of thiouracil was such that the speed of virus multiplication 



References p. 50/54 



