THE BIOCHEMISTRY OF PLANT VIRUSES 119 



there is no guanase activity, vast amounts of the unnatural analog may be 

 incorporated into the ribonucleic acids before much biological change is 

 evident, so that one must assume that azaguanine is relatively efficient in 

 performing the function of guanine, at least in these bacteria. 



One of the most remarkable features of the effect of purine analogs in 

 virus infection is shown by tobacco mosaic virus-infected plants treated 

 with 8-azaadenine. This compound is not incorporated into the virus nucleic 

 acid in detectable amounts but is converted by the virus-host system into 

 8-azaguanine, which is found in the nucleic acid of the virus as the nucleotide 

 (Smith and Matthews, 1957). The exact course of this remarkable trans- 

 formation is uncertain. An even more surprising synthesis accomplished by 

 the virus-infected plants is the production of 8-azaguanine from 4-(5)-amino 

 1-H-l : 2 : 3-triazole-5-(4)-carboxyamide, a synthesis which requires the 

 formation of the pyrimidine ring by incorporation of a one-carbon fragment 

 (C2 of the ring). 



A number of other analogous and related compounds have been tried as 

 antiviral agents, and the subject has been reviewed at length (Matthews and 

 Smith, 1955). Most of the compounds investigated have very little effect 

 or else the toxicity of the substances to the plants is too high for them to 

 be used for a successful cure of virus disease. 



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