Control of Cell Metabolism in Bacteria 299 



conditions of growth. The rate of synthesis of an inducible 

 enzyme can be stimulated as much as several thousand fold 

 upon the addition of its substrate or a related compound to the 

 medium (Cohn and Monod, 1953). The synthesis of other 

 enzymes can be influenced less directly, by a process called 

 sequential induction (Stanier, 1950). In sequential induction 

 the added compound (A, Fig. 1) induces an enzyme (E^) which 

 converts it to a second compound (B); the second compound 

 induces a second enzyme (Eg) which converts it to a third 

 compound (C); (C) induces a third enzyme (E^), etc. Induc- 

 tion thus influences the concentrations of enzymes that are 

 not directly involved with materials in the medium. By this 

 means, enzymes in metabolic pathways could be maintained 

 in sufficient excess to keep the concentrations of their sub- 

 strates at low levels. 



Another set of activities in bacteria are inducible; these 

 are mechanisms ("permeases") for the transport of sugars, 

 amino acids and other metabolites into the bacteria (Cohen 

 and Monod, 1957). Permeases are capable of specifically 

 building up very high concentrations (up to several per cent 

 of the dry weight of the cell) of their substrates inside the 

 bacteria. The function of inducible permeases and inducible 

 enzymes in regulation of metabolism will be considered below. 



Enzyme repression 



Repression, a mechanism just the opposite of induction, 

 controls the concentrations of some bacterial enzymes. In 

 repression, some small molecule inhibits the formation of an 

 enzyme; and often the latter is an enzyme that acts at an 

 early stage in the metabolic sequence of which the repressor is 

 a product (e.g. in Fig. 1, D might inhibit the process catalysed 

 by X^). Some early examples of repression, which were not 

 investigated extensively, were the repression of "methionine 

 synthase" by methionine (Wijesundera and Woods, 1953; 

 Cohn and Monod, 1953) of tryptophan desmase by trypto- 

 phan (Cohn and Monod, 1953), of p-galactosidase by galactose 

 (Cohn and Monod, 1953), and of the terminal transaminase in 



