RNA IN PLANT VIRUSES 29 



experiment of Gierer and Schramm (1956). By isolating tobacco 

 mosaic virus RNA by a very mild procedure (treatment with phenol), 

 they found that pure virus RNA is infectious. The purified active 

 RNA is quickly inactivated by ribonuclease and it soon loses 

 activity on standing, as a result of progressive denaturation. Pro- 

 teolytic enzymes have no effect on the infectivity and no proteins 

 could be detected by sensitive tests. 



When Gierer and Schramm's (1956) RNA gets into tobacco 

 leaves, it not only reproduces itself but it also synthesizes its protein 

 counterpart and produces virus of the strain from v/hich it origi- 

 nates. These experimental results are of far-reaching importance. Not 

 only do they demonstrate the essential role of RNA in protein 

 synthesis, but they also prove the genetic importance of RNA. In 

 plant viruses, as pointed out by Rich and Watson (1954), Jeener 

 (1956) and Gierer and Schramm (1956), RNA is the genetic deter- 

 minant just as DN A is in phages and in cells. 



It is a well-known fact that several "mutant" strains of tobacco 

 mosaic virus exist. According to analyses by Black and Knight 

 (1953), these strains differ in their amino acid composition rather 

 than in their content of purine and pyrimidine bases. More recent 

 work by Knight (1955) confirms that various strains of tobacco 

 mosaic virus are different in their terminal amino acid residues. On 

 the other hand. Price (1954) claims to have detected differences in 

 the composition of the RNA part of plant virus strains. In view of 

 these discrepancies and of the probable role of RNA as the genetic 

 determinant in tobacco mosaic virus, crucial experiments become 

 important. Such experiments have been made recently by Fraenkel- 

 Conrat (1956), who brilliantly succeeded in solving the problem. 

 Fraenkel-Conrat (1956) used the method outlined above and sepa- 

 rated the RNA and the protein parts of different strains of tobacco 

 mosaic virus. He then recombined the RNA from a given strain 

 with the protein of another and succeeded in producing experi- 

 mental "hybrids" between the two strains. When the virus was 

 recovered from the leaves which had been infected with these 

 "hybrid" virus particles, it was found that the lesions produced 

 belonged to the strain from which the RNA had been isolated. Of 



References p. 50/54 



