CONTROL BY REPRESSION 59 



used had glutamine synthetase activity. Since extensive purifica- 

 tion has not resulted in anv separation of the two activities, one and 

 the same enzyme seems to be involved (DeMars, 1960). Accord- 

 ingly, there is every reason to believe that this antagonism by glu- 

 tamine represents a case of repression entirely analogous to that 

 exerted on the formation of bacterial enzymes by the corresponding 

 "end products." 



Walker (1959) has investigated a presumable instance of repres- 

 sion of enzyme formation by an end product in an intact animal. He 

 found that the addition of creatine to the diet of rats brings about 

 a marked lowering of the level of kidney arginine-glycine transami- 

 dinase (which catalyzes a step in creatine synthesis). He pointed 

 out that the lowering of the transamidinase level might be related 

 to the turnover of the enzyme in the tissue examined. Other pos- 

 sible instances of enzyme repression in animals have been considered 

 by Auerbach et al. ( 1958 ) , Auerbach and Waisman ( 1958 ) , Potter 

 and Auerbach (1959), Knox (1960), and by McFall and Magasanik 

 ( 1960 ) . In a review of protein and nucleic acid turnover in relation 

 to biochemical differentiation, Mandelstam (1960) has discussed the 

 possible roles of enzyme repression and induction in the context of 

 development. The possibility that some hormones may have a re- 

 pressor-like action has also received some attention (Knox, 1960; cf. 

 Vogel, 1958). 



In conclusion, it would seem fair to say that the area of repression 

 has recently seen a great deal of progress. The interest in this area 

 may be due, in part, to the latter's position in the interdisciplinary 

 no man's land adjacent to various fields of chemistry and biology 

 and, in part, to the fact that repression can serve in constructs of 

 high precision or of broad scope. For the same reasons, the pace 

 set is likely to be maintained or increased in the future. 



References 



Adelberg, E. a., and H. E. Umbarger. 1953. Isoleucine and \ aline metabolism in 



EschericJiia coli. V. a-Ketoisovaleric acid accumulation. /. Biol. CJictn. 205: 



475-482. 

 Albrecht, a. M., and H. J. Vogel. 1960. Acetylornithine delta-transaminase: re- 



pressibilitv and other properties. Federation Proc. 19: 2. 

 Ames, B. N. 1960. Personal communication. 

 Ames, B. N., and B. Garry. 1959. Coordinate repression of the synthesis of four 



histidine biosynthetic enzymes bv histidine, Pwc, Natl. Acad. Sri. V.S, 45: 



1453-1461, 



