116 R. MARKHAM 



these circumstances was normal in its general properties. They also con- 

 sidered that the virus was as infective as the control virus grown on untreated 

 tobacco. The system which they used to demonstrate this was unusually 

 complex, but the general impression given by their data is that the virus 

 grown in the presence of thiouracil is considerably less infectious than is the 

 control material. Other workers have carried out more straightforward 

 comparisons, and the results indicate that the thiouracil-treated virus is of 

 similar infectivity to the control virus (Bawden and Kassanis, 1954). 



Jeener and Rosseels (1953) made the important observation that the 

 sulfur of the thiouracil was incorporated into the virus. They used S^^-labeled 

 thiouracil for this purpose, and used tobacco leaves infected with tobacco 

 mosaic virus floating on a liquid culture solution for growing the virus. 

 From their counts of radioactivity they estimated that as much as 20 % of 

 the uracil was replaced by thiouracil when the virus growth rate was reduced 

 to half. The identification of the thiouracil was somewhat tentative, and the 

 system which they used for the analysis actually decomposes thiouracil, 

 which is rather unstable to the usual hydroljTtic procedures. 



The fact that the thiouracil was actually incorporated into the virus 

 as such was confirmed by Matthews (1956) after a number of unsuccessful 

 attempts using whole plants. The amount of incorporation was about 3.5 % 

 of the uracil or some 70 residues per virus particle. It would appear that 

 little of the compound is incorporated unless the application is continuous, 

 as it is with floating leaves. The inhibitory effect of the compound is shown 

 in plants which have been sprayed, but under such conditions little or no 

 thiouracil seems to be incorporated. That the compound is incorporated 

 throughout the nucleic acid was shown by Mandel and associates (1957), 

 who separated out the smaU polynucleotide fractions of treated virus. 



Over half of the thiouracil was present in the ribonuclease digests as 

 thiouridylic acid or as the adenine- and guanine-thiouracil dinucleotides. 

 An interesting observation was that a quarter of the material was isolated 

 as a substance identified by comparison with authentic thymidine 3' : 5'- 

 diphosphate as thiouridine 3' : 5'-diphosphate, which should have much the 

 same electrical charge distribution as the former. If this compound is not 

 an artifact produced by enzyme contamination it would indicate that the 

 thiouracil-containing virus would contain 10-20 breaks in the individual 

 helices of nucleic acid. It is, however, evident that this work, like much of 

 the structural work on virus nucleic acid, could bear repetition. 



One very interesting observation has been recorded by Jeener (1957), 

 who used virus containing 14-18 % of thiouracil substituting for uracil. 

 He confirmed the earlier observations that this substitution had little or no 

 effect on the infectivity of the virus. Equal concentrations of virus of both 

 treated and control types were inoculated on to Nicotiana glutinosa; on 40 



