Control of Cell Metabolism in Bacteria 301 



metabolism. The description will be arbitrarily divided into 

 three parts: (a) uptake of nutrients, (6) intermediate steps of 

 metabolism and (c) formation of end products (large 

 molecules). 



(a) Uptake of nutrients. The initial steps of metabolism are 

 affected by processes which act so as to provide a plentiful 

 supply of nutrients. Permeases furnish a high intracellular 

 concentration of substrates, thus permitting subsequent 

 enzymes to function at more rapid rates than if they had 

 available only the lower concentrations of substrates in the 

 medium. Induction mechanisms tend to form large quantities 

 of permeases and of enzymes, and consequently provide for 

 high rates of metabolism. For instance, conditions have been 

 found which either do not permit formation of p-galactosidase 

 or else permit it to be formed only at the maximal rate in an 

 individual bacterium (Novick and Weiner, 1957). 



Mechanisms which limit the rate of utilization of nutrients 

 are not so well known, but there are good indications of their 

 existence. Diauxie provides an example: if the bacteria are 

 provided with two energy sources, one may be utilized in 

 preference to the other (a situation somewhat analogous to the 

 Pasteur effect). One imposes a limitation on the other. A 

 second case is observed when bacteria are transferred to a new 

 medium containing lactose as the sole carbon source; they 

 synthesize the enzyme for lactose utilization in preference to 

 the other proteins of the cell (Pardee, 1955; Rickenberg and 

 Lester, 1955; Mandelstam, 1957). This result could imply 

 that the capacity for synthesis of this enzyme was not fully 

 utilized under more favourable growth conditions. Mechan- 

 isms whereby nutrient limitations can be achieved are dis- 

 cussed later in this symposium (Magasanik, p. 334). 



(b) Intermediate steps of metabolism. Reactions of inter- 

 mediary metabolism can be divided into two classes in regard 

 to regulation. The fact that formation of metabolic inter- 

 mediates takes place in a series of steps means that each 

 reaction is dependent on the preceding ones for its supply of 

 substrates. Therefore, mechanisms for restricting the rates of 



