CONTROL BY REPRESSION 39 



appropriate regulatory allele (Vogel, 1953a; Gorini and Maas, 1958; 

 Gorini, 1960c; Albrecht and Vogel, 1960). It is noteworthy that, in 

 a derivative of the K-12 strain of E. coli, at least two enzymes of 

 arginine svnthesis, namelv acetylornithinase and acetylornithine 

 8-transaminase, can exhibit approximately coordinate derepression 

 (Vogel and Albrecht, 1960). 



Genetic factors, such as the o" allele of the /3-galactosidase sys- 

 tem (and possibly the characteristic allele of mutant strain liisG- 

 203), may be viewed as representing a switch that is always in the 

 'off' position; the o' type may represent a reduced affinity of the 

 operator or switch gene, or a product thereof, for the corresponding 

 repressor which is assumed to be produced under the control of the 

 z'-type gene (cf. Jacob et al., 1960a). 



The problem of the role of tlie i-type genes in the production of 

 repressors (or active repressors) has been examined by Pardee and 

 Prestidge ( 1959 ) . These authors performed experiments, such as 

 those of Pardee et al. ( 1959 ) illustrated in Fig. 2-5, except that pro- 

 tein synthesis, including yS-galactosidase synthesis, in the zygotes 

 was inhibited by 5-methyltryptophan from the moment of mating 

 the bacteria until reversal of the inhibition by the addition of tryp- 

 tophan 75 minutes later. At the end of this period, yS-galactosidase 

 was formed only when inducer was added. Conditions for induci- 

 bility were thus created under conditions of inhibition of protein 

 synthesis. The authors concluded that the repressor probably is 

 made directly by the gene rather than by an enzyme that itself is 

 produced under the control of the gene and, moreover, that the 

 repressor is not a protein. The repressor, or part of the active re- 

 pressor may, however, be a ribonucleic acid. Pardee and Prestidge 

 (1959) indicate, thougli, that a possible role of protein synthesis in 

 the production of repressor as governed by the i gene has not been 

 entirely ruled out. Jacob and Gampbell (1959) have obtained evi- 

 dence from a studv of Ivsoijenic bacteria that the formation of a 

 repressor may occur in the absence of protein synthesis. 



The repression concept has been successfully applied not only to 

 investigations of lysogeny but has been extended to considerations 

 of various episomic elements (Jacob et ah, 1960b) . This concept has 

 also been used in speculations on antibody formation (Szilard, 

 1960a, 1960b ) . Instructive analogies between yS-galactosidase forma- 

 tion and antibody synthesis have been discussed by Monod (1959a). 



