STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



113 



Strain 15t~ is almost but not completely blocked in the synthesis of 

 thymine; it may make 2 to 4 % of its normal requirement (Cohen and Earner, 

 1954). The enzymatic site of this block appears to be in the conversion of 

 deoxynridylic acid to thymidyhc acid (Flaks and Cohen, unpubHshed data), 

 and the cells accumulate uracil in amounts comparable to the thymine 

 deficiency. Uniquely, this block may be overcome by T2 phage infection, 

 since infected cells show a gross net synthesis of thymine (Earner and Cohen, 

 1954). 



-SC^"^ " SM -purines 



SM -thymine 



Fig. 



120 180 



(nnin) 

 51. The induction of thymine deficiency and "thjonineless death" in E. coli 

 grown in the presence of sulfanilamide (S) and metabolites (M) containing essential one 

 carbon fragments, including purines and thymine (Cohen and Earner, 1954, 1956). 



The thymineless organism is useful for the exploration of the interrelations 

 of RNA and protein synthesis. Under conditions of thymine depletion, 

 inhibition of KNA synthesis with 5-hydroxyuridine inhibited protein syn- 

 thesis proportionately (Ben-Ishai and Volcani, 1956). In amino acid defi- 

 ciencies both E.NA and protein syntheses w^ere inhibited, although a signi- 

 ficant synthesis of DNA occurred (Cohen, 1957). 



Multiple deficiencies, including those for thymine, purines, and a number 

 of amino acids, may be induced in nonauxotrophic strains of E. coli with 

 sulfanilamide (Rutten et at., 1950). The production of folic acid is inhibited, 

 thereby producing deficiencies in compounds containing the one carbon 

 fragments handled by folic acid derivatives. Growi:h may be restored 

 by the addition of these metabohtes to suLfanilamide. If thymine is speci- 

 fically eliminated from this medium, unbalanced growth is produced and 

 thymineless death ensues, as presented in Fig. 21. 



