94 MOLECULES, VIRUSES, AND BACTERIA 



ited by the amount of the DNA ( gene ) that specifies this enzyme? Or 

 is there some unknown control mechanism which hmits the maximum 

 rate of formation of an enzyme, independent of the number of gene 

 copies in a cell? To find the answer, investigation has been initiated of 

 the physiology and genetics of bacterial mutants that make exceptional 

 quantities of certain enzymes. Such mutants may be used to even 

 greater advantage in obtaining information about the mechanism of 

 protein synthesis and its control. In the usual cell many different pro- 

 teins are being made, and it is difficult to identify chemically or physi- 

 cally any component of the cell specifically involved in the synthesis of 

 a particular enzyme. But in a cell so specialized that it makes largely 

 only a single protein, such identification becomes possible. 



It would be particularly advantageous to have a situation in which 

 the enzyme made at a high rate is inducible, i.e., is formed only in the 

 presence of a specific inducer. We could then compare a bacterium that 

 makes the enzyme with a counterpart that does not. The two cells 

 could be identical in all respects except the features concerned specifi- 

 cally with the synthesis of this enzyme. This would allow an oppor- 

 tunity to identify those parts of a cell specifically associated with the 

 formation of a specific enzyme. 



The hope for positive results in such a study rests in part on the 

 assumption that changes in the cell which are associated with this syn- 

 thesis can be detected. If, for instance, the apparatus necessary for syn- 

 thesis of the enzyme is normally present in a cell and the only effect of 

 the inducer is to activate this apparatus, it will not be possible to find 

 any distinguishing feature in the synthesizing cell, aside from the 

 presence of the product enzyme itself. Likewise, the difference between 

 a synthesizing and a non-synthesizing cell will be difficult to detect if 

 the synthesizing apparatus constitutes only a small fraction of all the 

 similar material present. 



However, if positive differences between the cells can be found, 

 many important questions regarding the mechanism and control of pro- 

 tein synthesis may be answered. 



In preparation for such a search, techniques have been developed 

 for the isolation of bacterial strains which make very large amounts of 

 certain inducible enzymes. The present discussion is concerned with a 

 description of the techniques for the isolation of suitable strains and a 

 discussion of the properties of some of the strains that have been 

 obtained. 



Isolation of bacteria with high enzyme levels 



Two cases are already known in which bacteria apparently can 

 produce substantial quantities of a single enzyme. Melvin Cohn ( 1957 ) 



