160 D. Wright Wilson 



whether the nucleic acids are built up from small units de 

 novo or whether the bases may go in and out of the nucleic 

 acid without its complete disruption. The pyrimidine bases 

 of the DNA have always shown far less incorporation of radio- 

 activity than those of RNA. However, in the case of regener- 

 ating liver the incorporations in the DNA pyrimidines were 

 unusually high. 



We have studied other intermediates besides orotic acid 

 (Table VIII). I wish to report here an experiment carried 

 out in collaboration with a team under the direction of 

 Lemuel Wright (1951). Ureidosuccinic acid was fed to a 

 strain of Lactobacillus bulgaricus which required orotic acid, 



in amounts sufficient to obtain a rate of growth similar to 

 that brought about by orotic acid. The ureidosuccinic acid 

 had the same specific activity as did the orotic acid. Bacteria 

 which were grown in such a medium contained uracil and 

 cytosine having the same specific activities as they had when 

 the bacteria were grown with radioactive orotic acid. It 

 would appear that ureidosuccinic acid is a precursor of 

 pyrimidines in these bacteria. 



With S. S. Cohen (Weed and Cohen, 1951) we have studied 

 the' incorporation of radioactivity into bacteria and viruses. 

 See Table IX. When E. coli is grown in a medium containing 

 radioactive orotic acid, radioactivity is incorporated in the 



