CONTROL BY REPRESSION 47 



relatively insensitive to irradiation with ultraviolet light (Novelli, 

 1960). 



Apparenth', a concomitant formation of ribonucleic acid is not 

 necessarv for the svnthesis of ornithine transcarbamvlase: Rogers 

 and Novelli ( 1959 ) showed that the enzvme can be produced in a 

 mutant of E. coli under conditions of uracil starvation (Fig. 2-7). 

 Such enz\me synthesis was attributed by these authors to the pres- 

 ence of preformed synthesizing sites. Rogers and Novelli (1959) 

 also found that, even under conditions of uracil starvation, the syn- 

 thesis of ornithine transcarbamylase is repressible by arginine ( Fig. 

 2-7). 



Yates and Pardee (1957) had previously obtained evidence for 

 the derepressed formation of ( uracil-repressible ) enzymes of pyrimi- 

 dine synthesis in uracil-starved E. coli mutants. Recently, Kennell 

 and Magasanik ( 1960 ) investigated the derepressed synthesis of the 

 inosine 5'-phosphate dehvdrogenase of Aerohacter aerogenes, as a 

 function of the concentration of ribosomes in the cell. This enzyme 

 is repressible by guanine and shows derepressed synthesis under 

 conditions of guanine starvation (Levin and Magasanik, 1959) and 

 hence in the absence of any net synthesis of ribonucleic acid. Upon 

 control of the ribosome level through magnesium deprivation, Ken- 

 nell and Magasanik (1960) were able to demonstrate that the rate 

 of derepressed synthesis of the dehydrogenase is proportional to the 

 concentration of ribosomes in the cell. These authors concluded, 

 among other things, that ribosomes capable of producing the dehy- 

 drogenase apparentlv are present in the cell even when the dere- 

 pressed synthesis of the enzyme is not taking place. 



These indications that repressible enzyme synthesis can occur 

 without concomitant ribonucleic acid formation are, of course, highly 

 relevant to considerations of the site of repressor action. Support 

 for these indications comes, for example, from the extensive studies 

 on general protein synthesis of Magasanik et al. ( 1959 ) and Neid- 

 hardt and Magasanik (1960) with A. aerogenes. Consistent with 

 these studies in bacteria are the graphic results of Zalokar (1959, 



virtually abolished (top curve). Following the addition of the arginine and 

 uracil (at zero time), deoxyribonucleic acid synthesis, as judged by thymine in- 

 corporation, proceeds with a lag similar to that of /i-galactosidase formation 

 (bottom curve); in contrast, general protein synthesis and ribonucleic acid forma- 

 tion are initiated promptly. (Courfesy, R. B. Roberts.) 



