38 BAWDEN AND PIRIE 



ones are incapable of initiating infection. As with tobacco mosaic virus, 

 there is no evidence to show whether the small particles are more likely 

 to be stages in the synthesis or in the degradation of fully active 

 particles. 



Preparations of the tobacco necrosis virus made by the ultracentrifu- 

 gation of recently expressed sap are less infective than those made from 

 duplicate lots of sap allowed to age for two days before centrifugation, 

 although otherwise the properties of the preparations do not greatly 

 differ. This suggests that infectivity is a property acquired by many 

 particles during extraction or after they are extracted from infected 

 cells. The acquisition of infectivity has two equally plausible interpre- 

 tations. It may come from some terminal process which occurs in the 

 sap and adds an essential grouping or rearranges existing groups. Al- 

 ternatively, the virus particles may be liberated fully formed into the 

 sap but be combined with some cell constituents that act as inhibitors 

 of infectivity, and removal or destruction of the inhibitors may be 

 responsible for the activation. The second possibility has some in- 

 teresting consequences. Purified viruses combine to form complexes 

 wdth a range of substances, some of which are powerful inhibitors of 

 infectivity, and it is reasonable to assume that combination with some 

 specific cell constituents is also an essential step in the initiation of in- 

 fectivity. The inhibitors presumably act because they combine either 

 with parts of the virus that are concerned with this initial combination 

 or with the specific cell constituents. If virus particles were incom- 

 pletely dissociated from the cell components in combination with which 

 they are synthesized, inhibition of infectivity could be expected, for 

 there is no more simple or effective method of blocking a lock than to 

 leave a part of the key in it. Studying the extraneous material in 

 poorly infective virus preparations may prove a fruitful source of in- 

 formation about the mechanism whereby viruses are produced. 



There is no reason to consider that the three viruses mentioned 

 above are exceptional in causing the production of a range of sub- 

 stances, only some of which are infective. Nor is there any reason to 

 assume that the phenomenon is one peculiar to plant viruses. Work on 

 the complement-fixing antigens that are produced at different times 

 during infection with influenza virus, shows that material with the 

 antigenic specificity of the virus is produced before any infective par- 

 ticles can be detected (Hoyle 1948). Analogies could obviously be 

 drawn between this non-infective material and the non-infective 

 nucleo-proteins that carry the serological specificity of tobacco mosaic 

 virus or with preparations of the Rothamsted tobacco necrosis virus 

 whose infectivity increases as they are exposed to the environment of 



