80 Essays in Biochemistry 



constituent, uracil. In this interval, a mixture of radioactive ultra- 

 violet-absorbing substances were excreted into the medium. Three of 

 these have been isolated and were shown to be uracil, orotic acid, and 

 hypoxanthine. Their radioactivities per atom of C were identical with 

 the glucose used in the experiment, demonstrating that these compounds 

 were actually synthesized in toto while the cell was growing and dying. 

 Uracil was the major constituent excreted, and it may be suggested 

 that in this system this base or a derivative is the precursor which is 

 methylated to form thymine. 



It was observed that a thymine analog, 5-bromouracil, permitted a 

 five- to sixfold increase of turbidity as well as considerable synthesis 

 of both DNA and RNA in the absence of thymine. Indeed cell number 

 doubled, and then, despite a continuing DNA synthesis, death occurred. 

 On examination the cells were seen to be very long and we concluded 

 that the cells had synthesized an inadequate DNA, which, containing 

 bromouracil instead of thymine, was unable to support normal di- 

 vision. 5 



A cell in which cytoplasmic synthesis proceeds in the absence of 

 nuclear synthesis is a useful tool. It has been possible to demonstrate 

 not merely enzyme synthesis in such a system but the synthesis of a 

 new enzyme in response to the presence of an inducing substrate, i.e., 

 enzymatic adaptation. The enzyme tested was xylose isomerase, which 

 ratalyzes the equilibrium: 



D-xylose ^ D-xylulose 



Xylose isomerase is undetectable in cells grown on glucose. "When 

 15 T - grown on glucose was exposed to D-xylose in the absence of 

 exogenous thymine, the bacteria synthesized this enzyme in response 

 to the presence of the inducing substrate. The initial appearance and 

 rate of synthesis of the enzyme were similar to those observed in the 

 presence of thymine. Thus nuclear synthesis does not appear essential 

 to the phases of induction and synthesis. We have concluded that 

 the cytoplasm is the site of such activities. 5 Of course, having made 

 xylose isomerase in the absence of thymine, xylose was now used by 

 15 T - for unbalanced growth, and the cells died, that is, the cells com- 

 mitted suicide as a result of adaptation. 



Death of 15 T - in the absence of thymine does not occur until cyto- 

 plasmic growth has proceeded past some critical point. If, just before 

 this point is attained, thymine is furnished to the cells it is found that 

 the entire cell population divides synchronously, and a considerable 

 degree of synchrony is maintained for at least four cycles. When 



