S. SPIEGELMAN AND A. M. CAMPBELL 



mutants each of which was deficient in the abihty to synthesize a 

 single amino acid. These mutants were subjected to a "specific 

 starvation" by being grown in a medium in which the required 

 amino acid was present in Hmiting quantities, whereas all other 

 compounds were in excess. Immediately upon the cessation of 

 growth which attended the exhaustion of the amino acid, an 

 inducer of the beta-galactosidase was introduced. It was found 

 that little or no enzyme was synthesized by cells so treated, 

 despite the fact that they contained normal amounts of Pz. 

 Such cells do, however, form enzyme immediately upon the 

 addition of the amino acid they are unable to synthesize. These 

 results made it necesssary to abandon any interpretation of the 

 relation between Pz and the beta-galactosidase which involves a 

 direct, amino-acid-independent conversion of Pz into active 

 enzyme. The large number of amino acid auxotrophs em- 

 ployed in this study would suggest further that if Pz is indeed the 

 precursor, a considerable number and variety of amino acids 

 must be added to it before it is converted into active enzyme. 



Ushiba and Magasanik (98) employed essentially the same 

 approach in their study of the adaptive utilization of myoinositol 

 by mutants of Acetobacter aerogenes. The results obtained led 

 these authors also to the conclusion that the induced formation 

 of the enzymes they were studying involved extensive synthesis 

 from the amino acids. In a subsequent study, Rickenberg et al. 

 (63) reported similar experiments and results with E. coli 

 strain K12, 



IS THERE A PREEXISTENT COMPLEX PRECURSOR? 



The experiments cited thus far make unlikely any mechanism 

 of synthesis which presumes the conversion of a preexistent pre- 

 crusor into enzyme by a process which is independent of the free 

 amino acids. The only alternatives left are, either that there is 

 no preexistent complex precursor, or that one does exist but be- 

 comes active enzyme only after the incorporation of amino acids. 

 The most obvious experimental approach aimed at a choice 

 between these alternatives would appear to be the use of isotopic 



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