THE VARIETIES OF MACROMOLECULES IN EXTRACTS 

 FROM VIRUS-INFECTED PLANTS 



F. C. Bawden and N. W. Pirie 

 Rothamsted Experimental Station, Harpenden, England. 



Purified preparations of some plant viruses, for example, tomato 

 bushy stunt, show no signs of heterogeneity when subjected to cus- 

 tomary physical tests, and the properties and gross chemical constitu- 

 tions of preparations made at different times are also reproducible. 

 Despite this seeming appearance of uniformity, there is no reason to 

 consider that the preparations are homogeneous in the sense that every 

 particle has precisely the same chemical constitution and biological 

 activity. There is no method whereby such a uniformity could be dem- 

 onstrated and there are many ways in which particles could differ from 

 one another without any differences being detectable. The probability 

 that infective and non-infective particles could occur but be undetected 

 is obvious from the fact that inactivation by various methods does not 

 change the morphology of the particles or their gross chemical, phys- 

 ical and serological properties. 



To produce a single infection with any of the plant virus prepara- 

 tions so far studied, inocula must contain many thousands of particles. 

 This can be explained away by postulating that most of the particles are 

 wasted because they never encounter a susceptible site on the rubbed 

 leaves or that infection occurs only when a large number of iden- 

 tical particles enter a cell simultaneously, but it is also possible that 

 even the most infective preparations do not consist exclusively of in- 

 fective particles. The local-lesion assay method allows accurate com- 

 parisons of the relative infectivities of different preparations of a virus, 

 but there is no method of estimating the proportion of the particles in 

 any one preparation that is infective. Physical tests for detecting in- 

 homogeneity also are too insensitive for positive conclusions about the 

 uniformity of particles in purified virus preparations. They will fail 

 to distinguish between particles that differ in mass by i or 2%, and 

 it is worth stressing that 1% of the particle of even a small virus 

 weighs as much as a haemoglobin molecule. This limitation in the 

 precision of our physical methods, a limitation that is likely to persist 

 for many years to come, means that only heterogeneity can be demon- 

 strated, not homogeneity. 



There is now ample evidence that extracts of plants infected with 

 certain viruses do contain a mixture of related but different substances, 

 which are either specific to infected plants or occur in healthy plants 

 in much smaller quantities. Although many claims have been made 

 about the homogeneity of preparations of tobacco mosaic virus, this is 

 one with which such mixtures are readily demonstrated. B}^ following 



