56 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



but is converted to other substances that, possibly upon "activation," 

 cause the various repressions. Support for this view has come from 

 the discovery by Neidhardt (1960a, 1960b) of a mutant of A. aero- 

 genes in which gkicose fails to repress a number of enzymes whose 

 formation is glucose-sensitive in the parent strain; gluconate, how- 

 ever, is equally repressive for the mutant and the parent strains. 

 Magasanik and Bojarska (1960) have examined peculiarities in the 

 glucose metabolism of this mutant strain and concluded that the 

 repressive action of glucose may be exerted via gluconate. 



NH, 



H 

 HOOC-H^C-C-COOH 



NH 



Nucleic Acid <- N 



-N 

 I 

 RP 



NHj 



N^'S— N 



Hisf 



Fig. 2-9. The regulation of the cyclic interconversions of purine nucleotides. 

 See the discussion by Magasanik and Karibian (1960). The dashed lines inter- 

 secting arrows indicate the sites of feedback inhibition by guanosine 5'-phos- 

 phate (GMP), adenosine triphosphate (ATP), or histidine (Hist). The enzymes 

 corresponding to steps 2 and 8 are repressible by guanine; the enzymes cata- 

 lyzing step 6 and the conversion of the product of this step to imidazoleglycerol 

 phosphate are repressible by histidine. {Courtesy, B. Magasanik and D. Karibian.) 



