220 ERNST FREESE 



genie. For thir> reason tlicii- iiiuta<icnic cfft'el lias been alti'ihutcd to base 

 pairing mistakes that occur as a cons('(|uence of tlicii- incorpoi'ation into 

 DXA (Freese, 1959a). 



1. 5-Bnymourncil (/^f'l or o-Hroinodcoxj/uriflinc (Bl'dR) 



Figure 5A illustrates that BU can pair with adenine (A) as well as 

 thymine (T) can. When T is lacking, addition of BU (or better, BUdR) 

 to the growth medium leads to the (luantitative replacement of T by BU 

 in DNA. In most cases extensive death has been reported under such 



H 

 ^ N >-H O Br 



\9/56\ /45\ 



N— C4 iN H— N3 6C — H 



/ \3 2/ \2 1/ 



— C N-C C— N 



\ \ <i )c- 



Adenine 5-Bromouracil 



(normal amino state) (normal keto state) 



(A) 



\ p-^ p-\ 



N-C N— H N C— H 



/ \ / \ / 



— C N— C C— N 



\ \ / \ 



N-H O C- 



i ' 



Guanine 5-Bromouracil 



(normal amino state) (rare enol state) 



(B) 



Fig. 5. Base pairing of 5-bromoiiiacil (BU). A. Normal base pair of BU with A. 

 B. Rare ba.se pair of BU with G. 



growth conditions, but when phages (Stahl, 1961) or transforming prin- 

 ciple (Szybalski et al, 1960) are kept in the absence of light veiy little 

 death occurs. In other organisms BU may have side effects on vital cell 

 constituents different from DNA. Occasionally a base pair change in 

 DNA is induced when BU loses its hydrogen atom in the 1 -position and 

 pairs with guanine (G) instead of A (Fig. 5B). This ])airing "mistake" 



