Control of Cell Metabolism in Bacteria 303 



play a dual role in control of metabolism: they regulate the 

 formation of small molecules, and also provide for an econo- 

 mical formation of certain large molecules (which happen to 

 be enzymes). 



The syntheses of proteins and nucleic acids, processes in 

 which many metabohc pathways converge, provide outstand- 

 ing possibilities for metabolic control. Furthermore, since 

 these materials make up a large part of the organic material 

 of bacteria (perhaps 80 per cent), a means might be available 

 for the co-ordination of the bulk of the metabolism of the 

 bacteria. First, it must be remembered that proteins (or even 

 polypeptides) are synthesized only when all of their amino 

 acids are present (Spiegelman, Halverson and Ben-Ishai, '^ 

 1955). All amino acids whose pathways are controlled by 

 feedback or repression mechanisms would be formed at 

 rates which equal their rates of removal into proteins ; this, in 

 turn, would depend on the rate of supply of the least rapidly 

 available amino acid. 



Finally, it may be noted that the syntheses of proteins and 

 nucleic acids are closely related in bacteria: one does not 

 commonly find the synthesis of one of these macromolecuJes 

 continuing when a component of the other is withheld (Pardee, 

 1954; Spiegelman, Halverson and Ben-Ishai, 1955; Pardee and 

 Prestidge, 1956; Gros and Gros, 1958). Therefore, if one com- 

 ponent, say a nucleic acid precursor, were formed more 

 slowly than any amino acid, it would limit the synthesis of 

 most large molecules, and consequently of many of the small 

 molecules. 



Summary 



The small molecules in bacteria can regulate enzymes in 

 four ways : by stimulation of activity, inhibition of activity, 

 stimulation of formation (induction) and inhibition of formation 

 (repression). Products of metabolic sequences can act on 

 early reactions or enzyme syntheses (feedback), so that supply 

 is adjusted to demand for incorporation into large molecules. 

 The rates of many synthetic pathways could be co-ordinated 



