38 CONTROL MECHANISMS IN CELLULAR PROCESSES 



closely linked z and tj genes, constitutes with them a unit, called the 

 operon. The operator is thought to govern the activity of the genes 

 within its operon and to be susceptible to a repressor produced under 

 the control of a particular regulatory gene ( i ) . In the presence of 

 the repressor, the expression of the group of genes is considered to 

 be inhibited through the intermediate agency of the operator (Jacob 

 and Monod, 1959; Jacob et al., 1960a). The expression of a struc- 

 tural gene can be independent of the regulatory system to which it 

 is normally subjected, if the structural gene is linked to determinants 

 obeying another regulatory system ( Buttin et al., 1960 ) . 



An operator-type gene may also be involved in the control of the 

 formation of enzymes of the histidine pathway in S. typhimuriwn. 

 Ames et al. (1960) reported that mutant hisG-203, which maps 

 within the G locus ( Hartman et al, 1960b) corresponding to the first 

 enzyme of the histidine pathway, seems to lack not only the first 

 enzyme but also several, if not all, of the remaining enzymes of this 

 pathway. Accordingly, the mutation carried by hisG-203 seems to 

 give a "switch effect" suggestive of an operator-like locus (Ames, 

 1960). This result is particularly interesting in view of the finding 

 of Ames et al. ( 1960 ) , in conjunction with the genetic work of Hart- 

 man et al. ( 1960b ) , that the histidine genes appear to be linked in 

 a sequence that corresponds to the sequence of the enzymatic steps 

 in the histidine pathway ( cf . Demerec, 1956 ) . The suggested exist- 

 ence of an operator gene in this system would be consistent with 

 the aforementioned finding of coordinate repression of the enzymes 

 of histidine synthesis ( Ames and Garry, 1959 ) . It should be stressed, 

 though, that mutations within loci of the histidine path other than 

 G apparently can give position-type effects in that they seem to 

 bring about an impairment in the function of neighboring genes 

 (Ames et al, I960; Hartman et al, 1960b). 



A contrast to the sequential arrangement of the genes of histidine 

 synthesis is provided by the arginine case in E. coli. The structural 

 genes of arginine synthesis in strains B and K-12 of this species seem, 

 to a considerable extent, to be scattered in the genome (Gorini, 

 1960b) and yet are under the control of a single regulatory gene, 

 Rarg (Gorini, 1960a; Maas, 1960). In S. typhimuriwn, as in E. coli, 

 the genes governing arginine synthesis are not all closely linked 

 (Demerec et al, 1960). At least five, if not all, of the enzymes of 

 arginine synthesis are repressible in strains of E. coli carrying the 



I 



