280 HEINZ SCHUSTER 



rate of deamination of bases. In the experiments with TMV-RNA, a value 

 of somewhat over 3000 was found for N. This means that one-hit, that is 

 one deamination, per approximately 3000 nucleotides results in a loss of 

 infectivity of the entire molecule. Since the polynucleotide chain of TMV- 

 RNA is some 6000 nucleotides in length, this would mean that at least 

 half of all the nucleotides are necessary for the transfer of genetic informa- 

 tion. One might also say that there is a 50 % probability that one-hit would 

 inactivate an RNA molecule. Since all three amino bases are deaminated 

 at rates which differ very little from each other, it is not possible to decide 

 which alterations lead to inactivation. But from the value of N, it is clear 

 that a certain number of nucleotides can be deaminated without causing a 

 loss in biological activity. It is also conceivable that a deamination of cer- 

 tain bases or of a certain base does not result in inactivation but rather in 

 mutation. 



(2) Production of Mutants. Gierer and Mundry 119 • 12 ° succeeded in demon- 

 strating that HN0 2 treatment of TMV-RNA or intact TMV actually 

 results in the formation of mutants. TMV-RNA or TMV was incubated 

 with HNO2 under the conditions described above, and mutants were demon- 

 strated using the Java variety of tobacco. The systemic strain of TMV can 

 only produce chlorotic lesions on the leaves of Java tobacco. Spontaneous 

 mutations, which are often found, give rise to necrotic lesions. Thus, the 

 number of necrotic lesions on Java tobacco plants must be proportional to 

 the concentration of mutated virus particles. It is possible to determine the 

 total concentration of surviving infectious particles by counting the number 

 of necrotic lesions which are produced on leaves of Xanthi or Nicotiana 

 glutinosa plants. The number of mutants which are produced by HN0 2 

 treatment of TMV-RNA are shown in Table VIII. This is expressed as the 

 % necrotic lesions among total chlorotic plus necrotic lesions on Java 

 tobacco plants (see Table VIII, line 6). For any time of treatment with 

 HNO2 , this value lies well above the number of spontaneous mutations 

 which are found with untreated RNA. The inactivation of RNA with 

 HNO2 is exponential in time [see Eq. (5)]. The concentration of infectious 

 particles n at time t is given by the equation 



n = n e~ (/T (6) 



where n = concentration of infectious particles at time t = 0, t = time 

 for which the number of infectious particles has decreased to l/e, (37% of 

 n ). 



If the alteration of a single base is mutagenic, and if the average number 



119 A. Gierer and K. W. Mundry, Nature 182, 1457 (1958). 



l ™ K. W. Mundry and A. Gierer, Z. Vererbungslehre 89, 614 (1958). 



