226 



ERNST FREESE 



containing systems, including transfomiing principle (Litman and 

 Ephi-ussi-Taylor, 1959). 



Nitrous acid induces point mutations in T4 phages (Freese, 1959c; 

 Benzer, 1961) and in yeast, i.e., Schizosaccharomyces pombe (Gutz, 

 1961). 



The effect of the deaminations on the pairing properties of the differ- 

 ent bases is depicted in Fig. 9. One can conclude (Freese, 1959c) that 



^\ 



\ 



/ 



.c-c 



\ / 

 N— C 



/ \ 



— C N- C 



\ ^ 



N— H- 



/ 



H 



Hypoxanthine 



H 



H. ^x N— H- 



C ^ / 

 \ C-C 

 \ / \ 



N-C x\ 



/ \ / 

 — C N— C 



H 



/ 

 -N H 



\ / 



-N C— H 



/ 



Cytosine 



(a) 



— O H 



■H— N C— H 



\ / 

 C — N 



/ \ 



o c— 



/ 



Adenine 



Uracil 



H 



II 

 H ^ C 



C ^^ G 



III 

 A C 



T i 



(b) 



«x. 



N— 



■ N O 



\ / 



c— c 



/ \ 



— c 



C N— H 



Vc' 



o 



Xanthine 



H 



/ 

 H-N H 



\ / 



N C— H 



\ / 

 /C-N 



o c 



/ 



Cytosine 



(c) 



Fig. 9. Base pairing and base pair changes of deaminated bases. A is deaminated 

 to hypoxanthine (H) which now pairs with C in place of T. C is deaminated to 

 uracil (U) which now pairs with A in place of G. G is deaminated to xanthine (X) 

 which still pairs with C, but by one less hydrogen bond. 



