34. THE RIBONUCLEIC ACIDS OF VIRUSES 281 



TABLE VIII 



The Production of Mutants of TMV-RNA by Treating 

 the RNA with Nitrous Acid" 



a TMV-RNA was treated with 1 M NaN0 2 solution at pH 4.8 (22°C.). After time (, the RNA is diluted 

 and assayed. Untreated controls of RNA contain an equivalent amount of nitrite (pH 7). Time t [Eq. (6)] 

 is 18.5 minutes. Total infections are given as numbers of lesions (per leaf) on Xanthi, and as the sum of chlo- 

 rotic and necrotic lesions on Java; mutant infections as numbers of necrotic lesions per leaf on Java. [Taken 

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



of such mutations per RNA molecule in the time r is p, then the concentra- 

 tion m of such mutants is: 



m = n p(t/T)e~" T = npt/r (7) 



or in terms of the maximum value of m: 



w/m max . = e(t/r)e~ tlT (8) 



Figure 9 shows the production of mutants under different conditions with 

 TMV and with RNA derived from two strains. It should be noted that the 

 mutagenic effect on TMV and on its isolated RNA is equal when the frac- 

 tion of bases altered in RNA, measured by t/r, is the same. The alteration 

 of single nucleotides must be mutagenic since the production of mutants 

 also follows a "single-hit" curve. 



If one nucleotide out of about 3000 nucleotides is altered, 102 6% of all 

 the infectious particles have mutated (see Table VIII). That means that 

 the alteration of any out of 180 of the total 6000 nucleotides in the RNA 

 strand would be mutagenic. Nevertheless, it remains undecided which ot 

 the three types of chemical changes caused by nitrous acid (alteration of 



