INDUCED ENZYME FORMATION 



alysis designed to uncover the mechanism underlying the 

 "slowness." They came up with the surprising finding that the 

 vast majority (99.9%) of ^s"Cell populations were noninducible. 

 When plated on galactose-EMB test plates they yielded small 

 negative clones. If the one out of a thousand positives were not 

 simply mutants, a situation of signal significance would be 

 available, for it would mean heritably different enzyme-forming 

 capacity against a constant genetic background. This turned 

 out to be the case since fluctuation tests designed to determine 

 the nature of the origin of the positives were inconsistent with 

 a mechanism involving random mutation. The Poisson variance 

 obtained strongly suggested that contact with galactose was 

 necessary and induced in a small proportion of the cells a 

 heritable modification leading to the ability to form the enzyme. 

 Once acquired, enzyme-forming ability was transmitted in- 

 definitely from one cell generation to the next, providing galactose 

 was present. However, growth of the positive cells in the 

 absence of the substrate resulted after six or seven generations 

 in a mass reversion to the original negative phenotype. The 

 mass reversion feature virtually eliminated any mutational 

 interpretation of the phenomenon. 



A detailed examination of the kinetics of the reversion was 

 made. The data obtained were most simply explained by as- 

 suming that positives contained units necessary for enzyme for- 

 mation lacking in the negatives. These units or elements could 

 increase only in the presence of the inducer galactose. Growth of 

 positives in the absence of galactose leads therefore to a dilution 

 of the relevant units, and a point is finally reached at which cells 

 are produced possessing either none or an insuflRcient number of 

 these elements. 



The nature of the conclusions and the potential usefulness 

 of the system for the study of the formation of enzyme-synthe- 

 sizing units warranted an attempt to obtain more critical evi- 

 dence. A more directly interpretable analysis of the nature of 

 the reversion phenomenon was undertaken (79) by the serial 

 isolation and characterization of single-cell progeny produced by 



137 



