DIFFERENTIAL DISTRIBUTION ANALYSIS 



143 



acid. For this reason, the nucleic acid produced by thymine 

 auxotrophs of E. coli, grown in the presence of thymine, was 

 investigated and compared with that formed by the same organ- 

 isms when suppUed also with 5-bromouracil. This 6-keto- 

 pyrimidine is known to be accepted by certain bacterial cells as 

 an analogue of thymine^^'^^. The incorporation of the halo- 

 pyrimidine, which is essentially limited to cells unable to syn- 

 thesize thymine, is accompanied by many biological consequen- 

 ces. The cells suffer a gradual loss of viabiUty^^ and exhibit a 

 considerable increase of the rate of formation of mutants^^, 

 sometimes with preferred sites of particularly high mutability^^. 

 It has also been shown that bacteriophage T2 particles may retain 

 their infectivity, despite the uptake of a high proportion of 5- 

 bromouracil^^. 



The results of our first study with the thymine auxotroph, 

 strain I, were quite surprising^*: they seemed to indicate a very 

 severe distortion of the sequence characteristics of the deoxy- 

 ribonucleic acid in which about 35 mole % of thymine had been 

 replaced by 5-bromouracil*^. Further work with this and another 

 mutant strain has, however, failed to bear out this conclusion; 



Fig. 13. Chromatography of hydrolysates of deoxyribonucleic acid of 

 E. coli, mutant strain I (Preps. 3 and 4 in Table 32): (a) grown in the 

 absence; (b) in the presence of 5-bromouracil. Elution from DEAE-cel- 

 lulose with an increasing linear gradient of LiCl in lithium acetate buffer 

 (pH 5); other techniques as in Ref. 51. Total volume of eluent 1,350 ml; 

 7-ml fractions. Taken from a recent paper^o. 



References p. 159 



