GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 629 



et al. ( 1953a ) showed that virus synthesis was associated with a net in- 

 crease in protein content. This was due to the presence of virus nucleo- 

 protein as well as to excesses of both insoluble protein and the soluble 

 non-virus protein that occurs only during virus synthesis. It was sug- 

 gested that the appearance of virus was preceded by the synthesis of 

 an insoluble precursor which is converted into virus or some soluble 

 intermediate protein, and that virus formation is due to a diversion of 

 some part of the host's protein-synthesis mechanism and not to protein 

 degradation. 



Virus synthesis results in the occurrence in infected plants of a 

 multiplicity of proteins which are not found in healthy plants. These 

 have been reported for both rod-shaped and spherical viruses ( Bawden 

 and Pirie, 1945; Markham and Smith, 1949; Rice et al., 1955; Sinclair 

 et al., 1957 ) . Several anomalous or accessory non-infectious proteins ac- 

 companying synthesis of tobacco-mosaic virus have been isolated, and 

 considerable speculation has been placed on their role in virus syn- 

 thesis. Takahashi and Ishii (1952a) isolated a protein which appeared 

 as roundish aggregates and which they termed "protein X." They later 

 ( 1952b ) showed that this protein would polymerize to form rigid rods 

 which were quite identical in gross morphology with the virus particles. 

 Commoner et al. (1952) described a similar component, "protein B," 

 and suggested that it might be a precursor of tobacco-mosaic virus pro- 

 tein. In a later paper (Commoner et al., 1953b) three such accessory 

 proteins, all of which were devoid of nucleic acid, were described. All 

 were made to polymerize into virus-like rods, and all were serologically 

 related to one another and to tobacco-mosaic virus. 



Studies made with radioactive isotopes quite clearly indicate that 

 the anomalous proteins are not degradation products of the virus. 

 Jeener (1954) studied the incorporation of C^^ into the nucleoprotein 

 and the nucleic acid-free antigen of the turnip yellow-mosaic virus and 

 found that the nucleic acid-free protein always incorporated the la- 

 beled carbon faster than did the nucleoprotein. He reasoned that the 

 former was not a degradation product of the nucleoprotein: that the 

 protein and nucleic acid were synthesized independently and subse- 

 quently united into nucleoprotein. It was suggested that the nucleic 

 acid-free protein was the protein portion of the uncompleted virus. 

 Similar results were obtained with C^^ and tobacco-mosaic virus and 

 similar conclusions reached by Van Rysselberge and Jeener ( 1957 ) . 

 Matthews ( 1958 ) questioned that the nucleic acid-free protein ac- 

 companying the turnip yellow-mosaic virus was the immediate pre- 

 cursor of the virus protein, because of his finding that the ratio of virus 

 nucleoprotein to acid-free proteins was approximately two to one and 

 was maintained as such over a long period and over a wide range of 

 conditions. 



