CONTROL BY REPRESSION 43 



ings of Nisman and Fukuliaia (1960) would argue against, l)ut not 

 exclude, S-RNA as a component of the active repressor. 



The possibility that nucleic acid is a component of active repres- 

 sors has a happy analogy with the adaptor hypothesis of Crick 

 (1958) in reference to general protein synthesis. To date, however, 

 there is no conclusive evidence that repressors require any activa- 

 tion at all for their functioning. What seems to be quite clear is that 

 there are well defined genetic loci controlling repression, l^ut such 

 control need not entail compound formation between the repressor 

 and some other substance. Various other types of interaction be- 

 tween repressors and regulatory genes or the latter's products are 

 conceivable, and, in particular, a joint activity involving a repressor 

 and a regulatorv gene product without compound formation does 

 not seem to be in conflict with the known facts. An advantage in- 

 herent in tliis possibility would be that prompt relief from repres- 

 sion, when necessary, would be achieved through mere utilization of 

 the small-molecule repressor, without the need for removal or break- 

 down of an active repressor. 



Site of Repressor Action. In the regulator hypothesis, it was as- 

 sumed, as mentioned above, that functional repressors and inducers 

 act on enzyme-forming systems, i.e., at the level of the presumable 

 secondary templates (the genie templates being considered the 

 primary ones). A substantial body of evidence supports such a 

 view; recentlv, though, some authors have entertained the possibility 

 of an interaction of primarv templates with repressors. These con- 

 siderations were based on the results from continuous cultivation 

 experiments under repressive conditions (Cocito and Vogel, 1958; 

 Vogel, 1958), on coordinate repression (Ames and Garry, 1959), on 

 studies of immunity of Ivsogenic cells ( Jacob, 1960 ) , and on studies 

 of the operator gene (Jacob et ah, 1960a). However, as pointed out 

 by these authors, alternative explanations are possil)le, and in par- 

 ticular, operator-type genes could express themselves at the level of 

 secondary templates. 



As discussed in an earlier section, coordinate repression was first 

 described for enzymes of histidine svnthesis in S. typhimuriiim, and 

 the genes corresponding to these enzymes are all closely linked. In 

 contrast, the genes of arginine synthesis in this organism and in E. 

 coll are not all closely linked, and yet the enzymes of arginine syn- 

 thesis apparently can be subject to repressor action under the con- 



