Control of Bacterial Cell Growth and Composition 335 



in their environment. For example, Aerobacter aerogenes, the 

 organism used in our experiments, can grow readily in media 

 containing glucose, glycerol, mi/o-inositol, histidine, or any 

 one of many other compounds as the sole source of carbon and 

 energy. This versatility of micro-organisms depends on their 

 ability to convert compounds of different structure by means 

 of specific enzymes to the same essential metabolites ; however, 

 the total capacity of the cell for protein synthesis is Hmited, 

 and the larger the number of different enzymes which the cell 

 contains, the smaller will be the amount of each individual 

 enzyme. Repression of the formation of an enzyme by a 

 product of its action enables the cell to dispense with making 

 more of the enzyme than is needed to provide the product at 

 a level sufficient for optimal growth; consequently, the cell 

 can use its capacity for protein synthesis advantageously to 

 produce more of those enzymes whose products are in short 

 supply. 



It has long been known that the rate of formation of certain 

 enzymes is enormously increased when their substrates are 

 added to the growth medium. In general, enzyme induction 

 seems to have the same physiological significance as enzyme 

 repression: both mechanisms select, within the genetical 

 framework of the cell, the enzymic composition best suited 

 for growth in the particular environment. It is therefore 

 pertinent to consider the mutual relationship of these two 

 regulatory mechanisms. 



The constitutive repressible enzymes catalyse essential 

 steps in the biosynthesis of amino acids, purines and pyrimid- 

 ines. These enzymes are readily produced by mutants with 

 genetical blocks prior to the step catalysed by the repressible 

 enzyme (Vogel, 19576; Gorini and Maas, 1957; Yates and 

 Pardee, 1957). The substrate, of the repressible enzyme is 

 neither produced by these mutants nor provided in their 

 growth medium, and it would therefore appear that its 

 presence is not required for the formation of the enzyme. 



On the other hand, the study of the inhibitory effect exerted 

 by glucose on the formation of many inducible enzymes, has 



