VIII. VIRUSES AND INSECTS 179 



A virus which has been inactivated by an agent can occasionally 

 be reactivated when the agent is removed ; this may result from the 

 memory of the original structure. Numerous analogous phenomena are 

 known and recognized in general as reversibility of protein denaturation. 



Usually yeast cells cannot ferment galactose, but if placed in 

 contact with the sugar, they will subsequently acquire the enzyme 

 necessary to ferment the sugar. Such an enzyme is generally called 

 adaptive enzyme. The adaptive enzyme is extremely unstable in the 

 absence of the specific substrate, and as a rule the enzyme will come 

 to show a loss of the specific activity shortly after the substrate is 

 removed from the medium. 



Spiegelman (94) held the opinion that adaptive enzyme is produced 

 from the protein precursor by the action of the substrate and that 

 since the change of the precursor to the enzyme is reversible, the 

 enzyme will come back to the state of the precursor when the subst- 

 rate is removed. This is almost in accord with writer's opinion, 

 according to which the precursor is the protoplasm protein itself whose 

 structure is altered by the substrate to become complementary to the 

 substrate structure, thereby the specific activity as the enzyme is 

 revealed, but as the change is reversible, on the removal of the sub- 

 strate which have caused the change, the original structure is reco- 

 vered (12). 



In the case of allergic dermatitis, dealt with previously, it is 

 supposed that the protoplasm protein of skin tissue cells is altered 

 in its structure by some agent, the altered structure being able to 

 spread successively in protoplasm and even from cell to cell, but 

 that since the altered structure is unstable, having a high tendency 

 to return to its original state, the skin tissue will soon recover from 

 the disease when the agent is removed. The failure of the demon- 

 stration of any virus in such dermatitis is probably dependent upon 

 such a lability of the altered structure. 



Some workers claimed that the adaptation of yeast cells to a sugar 

 could be transmitted by a virus-like agent, but some other workers are 

 disinclined to accept this claim. Also in this case strong reversibili- 

 ty of the structure may render the detection of the "virus" difficult. 



Protoplasm protein changed in its structure by a virus seems like- 

 wise to be inclined to recover its original structure. The recovery 

 from virus diseases may be accomplished mainly owing to this reversi- 

 bility (12). The recovery of the original structure is nothing but the 

 disappearance of the virus itself. 



Now we shall return to the problem regarding the insect vectors. 

 As above mentioned, insect vectors which have once become infective 

 by feeding on diseased plant do not always remain infective for long 



