IX. THE REJUVENATION OF VIRUSES 193 



The presence of specific relation between insect and virus as well 

 as the prompt disappearance of the infectivity can readily be explained 

 by this concept. The efficiency of fasting of the insect upon the 

 infection of the virus may be attributed to the increase by the fast- 

 ing of the agent specifically acting upon the virus. It is fully ex- 

 pected that for the occurrence of the activation proper proportion of 

 the virus to the activating agent will be required as in the case of 

 antigen-antibody reaction. It is said that the optimum conditions for 

 transmission are to use insects that have fasted for at least one hour, 

 feed them on the diseased plant for about 2 minutes and then transfer 

 them immediately to healthy plants (39). Increasing the length of 

 time the aphids feed on the infected plant greatly reduces the number 

 of infections obtained, indicating the decreased activation due to too 

 much amounts of the virus. 



Argument has been advanced thus far concerning the second phase 

 of denaturation, whereas now we have to discuss the third phase 

 shown in III in Fig. 21. If the environmental condition, under 

 which protein molecules are present in the first state, is changed, 

 thereby the structure of the protein being rendered unstable, then the 

 protein will be changed in the structure to adapt itself to the new 

 condition. It is considered that such a change is commonly accom- 

 plished by the refolding of the peptide chain. 



Thus, if some protein is altered in its structure from form I to 

 III in Fig. 21 by an environmental change, the structure represented 

 by III must be stable under the new condition. Environmental change 

 can be brought about by various physical or chemical agents, and 

 hence, if some chemical agent causes the structural change from form 

 I to III, it will be said that the form III is stable in the presence of 

 the chemical agent. 



The shift of the form, however, may often fail to occur when the 

 environmental change gradually arises, just as water may be super- 

 cooled without being frozen, but when an adequate stimulus is applied 

 to disturb the structure of the protein, the shift will immediately 

 start to attain to the stable state just as super-cooled water is sud- 

 denly frozen by stir. For example, we have frequently observed that 

 phage samples having been inactivated by some unknown causes are 

 activated by such manipulation as the addition of inorganic salts or 

 heating to a proper temperature (103). Such a phenomenon can be 

 explained by assuming that the phage remained in an inactivated state 

 despite of the removal of the inactivating agent, and that the mani- 

 pulation served only to initiate the recovery of the original activated 

 state. 



In general, the mechanism of the reversible inactivation of physi- 



