42 CONTROL MECHANISMS IN CELLULAR PROCESSES 



nation with a moiety ( of small or large molecular size ) which may 

 have the character of a type of adaptor. The principal support for 

 the notion of active repressors comes from genetic experiments. The 

 existence of genetic loci (such as i, Ruy, or Pia.g) that control repres- 

 sibility, and may or may not be linked to the corresponding struc- 

 tural genes, is highly compatible with the view that such regulatory 

 loci govern the formation of active repressors and that the small- 

 molecule repressors (for example, arginine) cannot function until 

 they are combined with their activating moieties. Particularly strong 

 evidence for the activation of repressors is provided by the experi- 

 ments of Pardee et ah ( 1959 ) , taken together with those of Pardee 

 and Prestidge (1959). As pointed out above, these authors con- 

 cluded that in the ^-galactosidase system ( a ) a specific repressor is 

 formed and (b) the appearance of this repressor does not seem to 

 entail protein synthesis; they suggested that the functional repressor 

 may contain ribonucleic acid. On the basis of the studies by Monod 

 et al. ( 1951 ) on the specificity of induction, it seems reasonable to 

 assume that the functional repressor of yS-galactosidase synthesis 

 possesses a moiety that is structurally related to galactosides, al- 

 though such a moiety need not be a derivative of galactose itself. 



The activating moieties of repressors could be repressor-specific 

 soluble-ribonucleate (S-RNA) molecules. Such S-RNA-type mole- 

 cules may be different from the S-RNA species thought to partici- 

 pate in general protein synthesis, since a mutation in a regulator 

 locus does not seem to affect the over-all functioning of the cell. It 

 should be remembered, however, that the experiments of Pardee 

 and Prestidge (1959) did not eliminate the possibility of a role of 

 protein synthesis in the production of repressor, as governed by the 

 regulator gene. A result of Nisman and Fukuhara (1960) may also 

 be relevant in this connection: they made an in vitro study of /3-galac- 

 tosidase synthesis with a preparation derived from a constitutive 

 strain of £. coli and found /3-galactosidase formation to be stimu- 

 lated by a nucleic acid fraction of the same strain; when this nucleic 

 acid fraction was replaced by an analogous one derived from an 

 inducible, but non-induced strain, an inhibition of enzyme formation 

 resulted, which they considered to be repression, and, interestingly 

 enough, this inhibition was not relieved when the nucleic acid frac- 

 tion from the inducible strain was pretreated with ribonuclease. If 

 this inhibition of enzyme synthesis was indeed repression, the find- 



