S. SPIEGELMAN AND A. M. CAMPBELL 



tion. Ben-Ishai and Spiegelman (7) undertook such a study. 

 One of the most effective compounds found was 5-OH-uridine, 

 which the experiments of Roberts and Visser (64) suggest is able 

 to prevent the utilization of uracil for the synthesis of RNA. 

 The presence of as little as 5 /ig./ml. of this compound results in 

 virtual cessation of beta-galactosidase formation by E. coli. 

 Further, this inhibition can be achieved even if the OH-uridine 

 is introduced subsequent to the addition of inducer, at a time 

 when maximal rate of enzyme formation has been attained. 



Several illuminating facts emerged from these experiments. 

 One was that the OH-uridine could effect a complete inhibition 

 of beta-galactosidase formation at concentrations which had no 

 effect on over-all protein synthesis. The apparent greater 

 sensitivity of the beta-galactosidase-forming system suggests that 

 it requires a larger effective supply of RNA precursors than other 

 protein-synthesizing systems. A second fact of interest is the 

 ability of the OH-uridine to prevent enzyme formation even 

 after its onset. This would suggest that continued synthesis of 

 RNA is required for the uninterrupted production of enzyme. 

 The same conclusion is derivable from the observation (56,58) 

 that, unlike the previously noted experience with thymine-less 

 mutants, uracil-less mutants cease making enzyme immediately 

 upon the exhaustion of externally supplied uracil. 



COMPETITIVE INTERACTIONS AMONG PROTEIN SYNTHESIS 

 SYSTEMS FOR RNA PRECURSORS 



The marked response of the beta-galactosidase-forming 

 systems to 5-OH-uridine and its interpretation in terms of an 

 elevated requirement for RNA precursors suggest other types of 

 experiments for exhibiting this kind of interaction. E. coli cells 

 growing logarithmically in a synthetic medium with ammonia 

 as the sole source of nitrogen do not accumulate a detectable 

 internal pool of amino acids. The rate of protein formation is 

 apparently limited by the synthesis of amino acids, since an 

 immediate increase in growth rate follows the addition of an 

 external supply of amino acids. It would be expected that the 



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