STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 141 



It was discovered relatively recently that yeasts contain significant amounts 

 of free amino acids and that the depletion of this amino acid pool then 

 limited enzyme synthesis (Halvorson and Spiegelman, 1953). 



On the other hand, it was known quite early that inducible enzyme 

 synthesis required an energy source and was blocked by inhibitors of oxida- 

 tive phosphorylation, such as dinitrophenol, and other inhibitors, azide, 

 arsenate, etc. In gram-negative bacteria, subsequently discovered to contain 

 relatively small internal amino acid pools, exogenous nitrogen and carbon 

 sources were also found to be essential to enzyme synthesis. 



In yeast, the induced synthesis of a-glucosidase is inhibited by a wide 

 variety of amino acid analogs, and in most instances a parallehsm exists 

 between the inhibition of enzyme formation and growth (Halvorson and 

 Spiegelman, 1952). As noted earlier, the unfulfilled requirement for a single 

 amino acid iii a series of auxotrophic mutants of E. coli is sufficient to prevent 

 formation of /S-galactosidase (Monod et al., 1952). Thus, an active metabolism 

 and de novo protein synthesis seemed required in the production of new 

 catalytic units. 



In the case of ^-galactosidase of ^. coli, it was shown by Cohn and Torriani 

 (1952) that extracts of /3-galactoside-induced bacteria contained a new 

 antigenic activity, G2, associated with the enzyme. The new antigen was 

 capable of stimulating the production of precipitating antibody in rabbits. 

 However, noninduced cells contained a cross-reacting protein, P2, which is 

 nonenzymatic and nonantigenic in rabbits, and precipitates poorly with 

 antisera to G^ in the presence of Gz. An immunochemical analysis of the 

 formation of Gz and Pz in extracts of induced bacteria revealed a fall in Pz 

 almost concomitantly with the increase in enzyme Gz (Cohn and Torriani, 

 1953). However, the increase in enzyme was far greater than the faU in Pz- 



That P2 is not a precursor of Gz was clearly demonstrated by the S^^ 

 isotope experiments of Hogness et al. (1955), discussed in Section IV, G, 1 

 on protein turnover, wherein it was shown that less than 1 % of Gz could be 

 derived from a preformed cellular precursor. Rotman and Spiegelman (1957) 

 have presented comparable evidence with the same system, using C^*-labeled 

 cells. These data ehminate a protein-conversion hypothesis and it seems 

 possible that the situation may be represented as shown in formula (VIII). 



Amino acids 



(VIII) 



This system is formally analogous to that which has been proposed for 

 antibody and normal y-globuhn production in the mammal, since these 



