CONTROL BY REPRESSION 33 



ance of enzyme activity upon derepression represents de novo syn- 

 thesis of the enzyme protein. More direct evidence on this point 

 has been obtained by Yates and Pardee ( 1957 ) who showed tliat the 

 aspartate transcarbamylase activity found on derepression is accom- 

 panied by the selective synthesis of new protein having the elec- 

 trophoretic mobility of the transcarbamylase. Rogers and Novelli 

 (1959) have concluded that derepression of ornithine transcar- 

 bamylase also represents the synthesis of new protein. Particularly 

 graphic illustrations of new protein synthesis have come from recent 

 studies of enzymes that show quantitatively very large derepression 

 effects, such as aspartate transcarbamylase (Shepherdson and Par- 

 dee, 1960a, 1960b) and alkaline phosphatase (Horiuchi et al., 1959; 

 Torriani, 1960; Garen and Levinthal, 1960). Of relevance to de 

 novo enzvme synthesis upon derepression are the well-known studies 

 of ^-galactosidase induction (Monod and Cohn, 1953; Rotman and 

 Spiegelman, 1954; Hogness et al., 1955), since the induction of this 

 enz>ine has been concluded to represent release from an endoge- 

 nouslv produced repression (Pardee, Jacob, and Monod, 1958, 1959; 

 see below). 



General discussions of the basic features of repression can be 

 found in the highly informative reviews by Monod ( 1959b ) , Magasa- 

 nik et al. (1959), and Pardee (1959, 1960) and in an article by the 

 present writer ( Vogel, 1959 ) . 



Genetic Aspects 



In view of the above-mentioned finding that the induction of 

 ^-galactosidase represents a release from repression, the body of 

 genetic knowledge from this well-explored system can be considered 

 pertinent to the genetics of repression. Additional results in this 

 area have come from investigations of other repressible systems, 

 including enzymes of tryptophan (Cohen and Jacob, 1959) and ar- 

 ginine (Gorini, 1960a; Maas, 1960) metabolism, as well as alkaline 

 phosphatase (Echols et al., 1960) and tyrosinase (Horowitz et al., 

 1960). For these systems, and for /3-galactosidase (Jacob and 

 Monod, 1959), enzyme synthesis appears to be under the control of 

 (a) "structural" genes, which are thought to contain information for 

 the structure (particularly the amino acid sequence) of the enzyme 

 involved, and of (b) regulatory genes governing the expression of 

 the structural ones through the agency of a repressor. 



