92 K. MAEKHAM 



1. Cucumber Virus 3 {and 4) 



The viruses called by these odd names were origmally kiiowii in England 

 as common cucumber mosaic, a name now given to the aphis-borne cucumber 

 mosaic virus, and they were evidently quite widespread. As a disease of 

 economic importance cucumber virus 3 has almost vanished, although an 

 epidemic was caused by it in Denmark quite recently (H. Ronde-Kristensen, 

 private communication). In England and in America it is only a curiosity 

 maintained among laboratory stocks. 



Cucumber virus 3 is very limited in its host range, being confined to a few 

 cucurbitaceous plants. It was only after its purij&cation by Bawden and 

 Pirie (1937b) that it was realized to be related to the tobacco mosaic virus. 

 In physical properties this virus resembles the tobacco mosaic virus in 

 almost every respect. The only gross difference lies in that the interparticle 

 spacing observed by X-ray diffraction is slightly smaller than in the tobacco 

 mosaic viruses (Bernal and Fankuchen, 1941a). As it is now known that 

 this distance is not necessarily related to the actual width of the particles, 

 and as the X-ray diffraction patterns obtained from the oriented virus 

 resemble those of the type tobacco mosaic virus closely (Franklin, 1956b), 

 there is every reason to suppose that the viruses are very similar. Although 

 tobacco mosaic viruses do not in general infect cucumber plants, it is possible 

 to show an interference in infection between cucumber virus 3 and tobacco 

 mosaic virus on cucumber cotyledons (Rochow, 1956). This is a criterion of 

 fairly close relationship between viruses, while the serological relationships 

 of the two viruses are marked. 



Cucumber virus 3 is unique among tobacco mosaic type viruses in that 

 it contains no sulfur and hence has no cysteine or methionine. It also has no 

 histidine (Knight, 1942, 1949, also Knight and Stanley, 1941). The trypto- 

 phan content is very low, being only one-quarter of that in the tobacco 

 mosaic \drus (Knight, 1947). The implications of this observation have been 

 discussed earlier, and it appears that the subunit of the protein part of the 

 virus is twice the size of the (apparent) subunit in the tobacco mosaic virus. 



The nucleic acid has an appreciably different base ratio (adenine, 1.04; 

 guanine, 1.03; cytosine, 0.73; and uracil, 1.19) from that of the type virus. 

 The predominant amino acid released by carboxypeptidase from the protein 

 is alanine (Knight, 1955a), and not threonine, which is found in a number of 

 other strains of the latter virus (most of which were in fact very closely 

 related, having been isolated from laboratory stocks). Largely on the basis 

 of these observations. Knight (1955b) has suggested that the virus should 

 not be regarded any longer as one of the tobacco mosaic viruses. This is, 

 of course, purely a matter of definition or even of personal taste. The virus 

 is so obviously related biologically and physically to tobacco mosaic viruses 

 that the reasons for excluding it would seem rather arbitrary. It is interesting 



