CONTROL BY REPRESSION 



55 



erned througli a combination of feedback inliibition and repression 

 of isocitratase by succinate, as indicated in Fig. 2-8. 



A revealing case of the interplay of multiple feedback inhibitions 

 and repressions can be seen in the purine nucleotide system de- 

 scribed by Magasanik (1960). The regulation of the purine nucle- 

 otide intercon versions (Fig. 2-9) includes (a) feedback inhibitions 

 by guanosine 5'-phosphate and adenosine triphosphate at steps 8 

 and 10, respectively, and (b) repressions by guanine, as discussed 

 above, at steps 2 (inosinicase) and 8 (inosine 5'-phosphate dehv- 

 drogenase); additionally, histidine can produce a feedback inhi- 



ISOCITRATASE- FORMING SYSTEM 



AMINO ACIDS 



(REPRESSION) 



-MSOCITRATASE 



-(INHIBITION)- 



ISOCITRATE 



< 



SUCCINATE 



GLYOXYLATE 



Fig. 2-8. Repression and feedback inhibition of the isocitratase of Micro- 

 coccus denifrificans. {Courtesy, H. L. Kornberg J. F. Collins, and D. Bigley.) 



bition at step 6, as well as repression of the enzymes catalyzing the 

 steps from adenosine triphosphate to imidazoleglycerol phosphate. 

 Both types of regulatory mechanisms thus contribute to the great 

 precision with which the purine nucleotide system is controlled 

 (Magasanik, 1960). 



The "Glucose Effect." Evidence is available that the formation 

 of a number of enzvmes, especially inducible ones, is antagonized 

 by glucose or metabolic products of glucose. This type of antago- 

 rjism, frequently termed "glucose effect," appears to be enzyme re- 

 pression (Magasanik et al., 1958, 1959; Colin and Horibata, 1959a, 

 1959b; Magasanik and Bojarska, 1960; Neidhardt, 1960a, 1960b). 

 In view of the pivotal position of glucose in metabolism, instances 

 of this effect would seem to represent repressions of particular sig- 

 nificance to the control of cellular function. 



It has been suspected for some time that, in the glucose effect, 

 glucose itself is not the functional repressor (Magasanik et al., 1958) 



