44 CONTROL MECHANISMS IN CELLULAR PROCESSES 



trol of a single regulatory gene. In the arginine case, therefore, the 

 picture of repressor action at the primary-template level is somewhat 

 less inviting than it is in the histidine case. The enzyme-forming 

 systems of the arginine pathway may thus each have sites sensitive 

 to the same repressor ( cf . Jacob et al., 1960b ) . Alternatively, the pos- 

 sibility exists that the enzyme-forming systems, despite the lack of 

 close linkage of all the corresponding genes, may be physically asso- 

 ciated and jointly susceptible to repressor action. In the case of 

 enzymes, such as tryptophan synthetase, that consist of more than 

 one protein component, each of which is repressible (Yanofsky and 

 Crawford, 1959; Yanofsky, 1960), it would appear especially likely 

 that the protein-forming systems producing the two components are 

 in close juxtaposition. Drs. Suskind and Yanofsky (Chap. 1) have 

 reviewed their very revealing studies on the formation of tryptophan 

 synthetase in E. coli and N. crassa. 



Further arguments in favor of repressor and inducer action at the 

 secondary-template level can be based on such findings as the con- 

 stant differential rates of induction of /3-galactosidase in cryptic ( per- 

 mease-deficient) strains of E. coli (Herzenberg, 1959; cf. Monod, 

 1958) and as the generally observed promptness of onset of repres- 

 sion, derepression, and induction. The virtually immediate onset 

 of repression of acetylornithinase, for example, indicates a prompt 

 cessation of derepressed enzyme synthesis that is compatible with an 

 effect of the repressor on the functioning of existing secondary tem- 

 plates but that would seem to be less compatible with an exclusive 

 eflect on primary templates. 



Repression and Nucleic Acid Metabolism. These thoughts re- 

 garding templates now lead to further considerations on the relation 

 of repression and induction to nucleic acid metabolism. Recent gen- 

 eral discussions of this topic have been presented by Magasanik et al. 

 (1959), Neidhardt and Magasanik (1960), Chantrenne (1958a, 

 1958b ) , and Maal0e ( 1960 ) . A comprehensive coverage of ribonu- 

 cleic acid metabolism is provided by the thoughtful article of Dr. 

 Zalokar (Chap. 4). 



Important advances in this area have been made as the result of 

 investigations of ribosomes in E. coli, which appear to contain the 

 secondary templates of this organism. The work of McQuillen et al. 

 (1959), Bolton et al. (1959), and Roberts (1960), for example, has 

 shown that ribosomes are indeed active sites of protein synthesis, 



