MACROMOLECULES 37 



(Schramm & Miiller 1940: Miller and Stanley 1942), although an alter- 

 native explanation of this may be that the substitutions are reversed by 

 the host cells before infection occurs. 



During the course of infection, too, it is likely that a range of 

 similar but not identical substances is S3rnthesized. Luria has suggested 

 that the synthesis of virus particles is the assembling together of macro- 

 molecular substances each of which is synthesized separately in infected 

 cells. If this is so, incomplete or faulty assemblies are sometimes to be 

 expected, and the material with the general characters of the virus but 

 lacking infectivity may be examples of such phenomena. Particles of 

 such material fall short of the full stature of virus particles, but the 

 fact that they are non-infective does not necessarily mean they are 

 devoid of activity in the cells where they were produced. The evidence 

 with irradiated bacteriophages suggests that two particles in which dif- 

 ferent activities have been destroyed may together initiate infection 

 though neither can alone, and non-infective particles of plant viruses 

 may also have some effects for which as yet we have no method of 

 testing. 



That infection leads to the production of more than one kind of 

 anomalous protein is also suggested by work on turnip yellow mosaic 

 virus (Markham & Smith 1949) and the Rothamsted tobacco necrosis 

 virus (Bawden & Pirie 1945 a: 1950). Purified preparations of turnip 

 yellow mosaic virus contain particles all of similar size, which have the 

 same isoelectric point and electrophoretic mobility, contain the same 

 antigens and which crystallize in the same form. Only 80% of the 

 particles, however, seem to be nucleoprotein, the remainder being an 

 apparently similar protein but free from nucleic acid. The ability to 

 initiate infection appears to be restriced to the nucleoprotein, but there 

 is no evidence to show whether all or only a proportion of the particles 

 can do so. Extracts from plants infected with the Rothamsted tobacco 

 necrosis virus also contain at least two kinds of specific particles. Both 

 these appear to be nucleoprotein, one being about twice the diameter of 

 the other. Apparently homogeneous preparations of the smaller par- 

 ticles can be made, but these are, weight for weight, less infective than 

 inhomogeneous preparations containing large particles. Mixed prepa- 

 rations can be fractionated by differential ultracentrifugation and by 

 precipitation at pH 4 in the presence of some normal leaf proteins. The 

 material that sediments more readily and precipitates at pH 4 carries 

 more infectivity than the other, but has less serological activity. The 

 large particles could be aggregates of the smaller ones, though this has 

 not been demonstrated. Nor is it proven that infectivity is a property 

 specific to the large particles, though it is clear that many of the small 



