118 S. S. COHEN 



F. Syntheses with Abnormal Metabolites 



The pathological systems described above provide excellent materials for 

 the study of the specificity of polymer syntheses. As noted in the case of the 

 carotenoidless Rhodopseudomonas spheroides, the carotenoid precursor, 

 phytoene, is accepted as a carotenoid replacement in the synthesis of 

 chromatophores to produce an altered and pathological cell structure. In the 

 case of the thymineless bacteria, a number of variations on this possibihty 

 have been observed. 



A synthetic analog of thymine, 5-bromouracil (BrU), permits the synthesis 

 of bacterial DNA containing BrU (Dunn and Smith, 1954; Zamenhof and 

 Griboff, 1954a,b). This occurs in the absence of thymine and a 6-fold 

 accumulation of DNA and RNA then may be obtained (Cohen and Barner, 

 1954). A single cell division is permitted in the presence of BrU, but the cells 

 then lose the power to multiply and form giant filaments (Cohen and Barner, 

 1954, 1956). The DNA containing a massive dose of BrU in strain 15t_ is 

 evidently incapable of fulfilhng the fmiction of DNA containing thymine. 

 However, if cells are grown in the presence of both thymine and BrU, the 

 cells do not die and contain both bases in their DNA (Zamenhof et al., 

 1956a,b). 



T2 phage has been grown in the presence of BrU in E. coli rendered 

 thymine- deficient m a sulfanilamide-containmg medium. Phages are pro- 

 duced which contain BrU; for the most part these are inactive. However, 

 Litman and Pardee (1956) have observed that a high proportion of mutants 

 (15 %) are obtained among the surviving progeny. It was reported that in 

 one such preparation, m which aU of the thymme had been replaced by BrU, 

 9 % of the total phages produced were still infectious. 



Chlorouracil and iodouracil are also capable of replacing thymine in DNA. 

 Other apparent thymine analogs are not replacements, but nevertheless 

 prevent the insertion or synthesis of thymine. These include 5-aminouracil 

 (Duncan and Woods, 1953) and 5-fluorouracil. When apphed to E. coli, 

 5-aminouracil caused the accumulation of A'^-methylammo purine in bacterial 

 DNA, as if the methyl group originally involved in thymine synthesis were 

 redirected to a new acceptor (Dunn and Smith, 1955). Chromosome breaks 

 can be produced by high concentrations of uracil, which appears to act like 

 a thymine antagonist under these conditions (Deysson, 1952). Both the pro- 

 duction of chromosome breaks by urethane, an inhibitor of pyrimidine 

 biosynthesis, and its carcinogenic action in mice are inhibited by thymine 

 (Rogers, 1957). The action of urethane in the production of pidmonary 

 adenomas is also potentiated by aminopterm, a foHc acid antagonist, which 

 inhibits thymine synthesis. Thus, it would appear that the presence of these 

 compounds leads to the formation of thymine-deficient chromosomes. The 



