282 



HEINZ SCHUSTER 



Fig. 9. The production of mutants of TMV by reaction of TMV or its nucleic acid 

 with HNO2 . The dependence of the concentration of mutants m (number of necrotic 

 lesions on Java tobacco relative to maximum value m x ) on the time t of incubation 

 with nitrous acid, related to t (the average time for one lethal conversion of a base 

 per RNA molecule); continuous line = single-hit curve, broken line = double-hit 

 curve. [A. Gierer and K. W. Mundry, Nature 182, 1457 (1958).] 



adenine, guanine, or cytosine) are mutagenic and how the chemically al- 

 tered RNA is related to its progeny. In the case of a conversion of cytosine 

 to uracil, it is conceivable that the altered RNA undergoes identical re- 

 production in the host cell. 



It is surprising that such a large number of mutants have the same phenotypic 

 expression of necrotic lesions on the given host (Java). This means that a cell of this 

 host is in a position to react with 180 different mutants in such a fashion that the 

 phenotypic effect (local necrotic lesions) is identical in all cases. The 180 mutants 

 differ from one another, however, in that each is produced by deamination of a given 

 nucleotide, and these nucleotides are located at different positions in the nucleotide 

 chain. A variety of other mutants are also produced by nitrous acid. In order to de- 

 tect them, individual lesions produced by RNA treated with HN0 2 were isolated 

 from Xanthi tobacco and assayed on Java and Samsun plants. Half of the lesions 

 which were reinoculated led to a variety of altered symptoms. The altered symptoms 

 proved genetically stable in a transfer experiment. 



The only possible reactions with HN0 2 are adenine — > hypoxanthine, guanine — » 

 xanthine, and cytosine — * uracil, and it may be that any one of these or perhaps only 

 the alteration of the naturally occurring bases, cytosine — > uracil may lead to muta- 

 tion. Now, if a mutant can only arise when the quantitative ratio between amino 

 bases and hydroxy bases changes, it should not be possible to produce RNA mutants 

 by other chemical transformations. Therefore, RNA which had been methylated with 

 dimethyl sulfate was also tested on Xanthi and Java plants. Even after an inactiva- 

 tion of greater than 90% of the RNA, it was not possible to observe necrotic lesions 

 on Java plants. 121 It would be worthwhile to see if any other changes in the nucleotides 

 of RNA lead to mutagenesis. It may be especially interesting to see if the unnatural 

 bases, azaguanine, thiouracil, and fluorouracil, which are incorporated into virus 

 RNA during the multiplication of the virus, produce mutations. 



121 H. Schuster and K. W. Mundry, unpublished work (1958). 



