S. SPIEGELMAN AND A. M. CAMPBELL 



Pollock (62) has observed interesting differences between 

 induced and noninduced cells in terms of sensitivity to ultra- 

 violet. He found that prior to the onset of penicillinase forma- 

 tion in the presence of inducer, enzyme synthesis is extremely 

 sensitive to inactivation by exposure to UV. Once, however, 

 enzyme formation has commenced, it becomes increasingly more 

 resistant to this type of inhibition. Halvorson and Jackson (31) 

 showed an exactly similar situation in the induced formation of 

 alpha-glucosidase by S. cerevisiae. 



These are findings which are not unexpected if, as seems 

 reasonable, empty templates are more sensitive to inactivation 

 by UV than those combined with protein in a complex stabilized 

 by inducer molecules. 



Finally, another difference between the induced and non- 

 induced state has been observed (84) in terms of response to 

 amino acid analogues. It has already been noted that it is pos- 

 sible to stop enzyme formation by means of amino acid ana- 

 logues. However, this inhibition is effective only if one adds the 

 amino acid analogue in the lag period of enzyme formation, 

 prior to the appearance of new enzyme molecules. If the induc- 

 tion is allowed to proceed to the point where enzyme is already 

 being formed, suppression of enzyme synthesis with the antag- 

 onist becomes increasingly difficult (84). We can explain this 

 apparently puzzling finding quite readily in terms of the model 

 just proposed. In noninduced cells the analogue is competing 

 with its homologue for an unoccupied position on the template. 

 It is not surprising that the competitive situation for the antag- 

 onist should be markedly modified when the empty templates 

 are filled with enzyme, for of necessity homologous amino acids 

 now sit bound at, or close to, the sites of competition. 



Our discussion of the biochemical, kinetic, and genetic in- 

 formation available on enzyme induction has led us to postulate 

 three components of the enzyme-forming system, inducer, pro- 

 tein, and template. In any given case the first two can be 

 reasonably well defined in terms of known chemical entities. 

 We turn now to the question of the identification of the chemical 

 nature of the third member of the triad, the template. 



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