THE BIOCHEMISTRY OF PLANT VIRUSES 



117 



leaves the counts were 5392 against 5189 lesions, which is as good as equal. 

 On the other hand, when tobacco leaves were inoculated with equal amounts 

 of the same material, Jeener found that the multiplication rate was much less 

 in the thiouracil-treated virus than it was in the control, the yield in his 

 experiments being only about one-third. This strange phenomenon has not 

 been investigated by other workers so far, but would really appear to deserve 

 further examination. Jeener explains his results by assuming that the virus 

 rod contains several infectious submiits, and that most of these can be 

 inactivated by incorporation of thiouracil, but that one or a small number 

 may survive, and then multiply at a reduced rate. This argument is not 

 unequivocal, and the reduced rate of multiplication might, for example, 

 be caused by an increased affinity of the virus for its protein coat, a possibility 

 which might exist because of the increased acidic strength of the enolic 

 groupings of thiouracil. Of course, as it is emphasized elsewhere, it is imposs- 

 ible to be absolutely certain about anything concerning viruses at the present 

 time. It may even prove that the protein of thiouracil-treated virus differs 

 from that of the untreated virus. This has not been investigated. 



A compound allied to the last one is 5-fluorouracil, which differs from the 

 natural base only in having a fluorine atom replacing a hydrogen atom 

 (Heidelberger et ah, 1957). This substance, whose structure is shown in 

 Formula V, 



O 



/ 

 H— N 



-H 



t/ 



5-Fluoroiiracil 



H 



(V) 

 possesses a very low-grade inhibitory effectiveness for the virus, the con- 

 centrations needed being lO-^-lQ-W (Davern and Bonner, 1958; Gordon and 

 Staehelin, 1958). These concentrations are highly toxic to tobacco leaves. 

 The quantity of fluorouracU incorporated is high, about 47 % of the uracil 

 being replaced by its analog. The only other halogenated uracU likely to 

 enter the ribonucleic acid of a virus is 5-chlorouracil, but this has not yet 

 been investigated in such systems. 



A. Azaguanine 

 Azaguanine is of interest as the first compound of an unnatural type to 

 be isolated from nucleic acid (Matthews, 1953) in the form of nucleotides, 



