THE BIOCHEMISTRY OF PLANT VIRUSES 93 



to note, too, that the N-terminal end of this virus is also blocked by an acetyl 

 group, and so the "N-terminal" sequence is somewhat 'reminiscent of that 

 of the tobacco mosaic virus. It is reported to be (for Cucumber 4) iV-acetyl 

 alanine, tyrosine, asparagine, proline, isoleucine, threonine, serme — (Narita, 

 1958a). 



2. Other Strains 



The ribgrass virus has been discussed earlier in this section, while chemical 

 differences existing between strains of tobacco mosaic virus are dealt with 

 elsewhere. 



VII. The Tomato Bushy Stunt Virus 



This virus, which has some claim to fame as being the first virus to be 

 crystallized in the form of three-dimensional crystals, was origmally isolated 

 from a tomato plant which was found infected under natural conditions 

 (Smith, 1935). This one isolate is the only one which has been worked with 

 by workers in various countries, largely because the disease was never again 

 recorded until a few years ago, when it was found to be widespread in Italy 

 (Gigante, 1955). This fortunate occurrence, the isolation of a stable virus 

 from a single source, has had the happy result that all workers have been 

 dealing with one virus, or at the least, virus which came from one original 

 source. Of course, there is the difl&culty that only one strain has been available 

 for investigation, but this trouble has now been bypassed to some extent 

 with the availabihty of the Italian isolates, which seem to differ somewhat 

 from the original type strain in their composition. 



Although from its name one might imagine that its preferred host was 

 tomato, a much more suitable host for propagation is Datura {tatula or 

 stramonium) (Stanley, 1940), because the latter is more readily infected and 

 is less susceptible to chance infection with tobacco mosaic. It may also be 

 grown on cucumber, tobacco, beans, cowpeas, and a number of other plants. 

 These last plants give only local infections under normal conditions, so that 

 the yield is not high, 



A. Purification 



The original purification was made by Bawden and Pirie (1938b) who 

 clarified their sap (from tomatoes) by heating to 60°C. This step, which was 

 known from the early observations of Smith (1935) to cause the inactivation 

 of the virus, resulted in a great loss of infectivity, but little if any loss of 

 material. This very fact, in these days when virus nucleic acids are being 

 used for transmission, makes one wonder what can possibly happen to the 

 virus under these conditions to cause its inactivation. Indeed this virus loses 



