REGULATION 139 



non-induced cells are grown in a medium containing glucose and a little 

 ^-galactoside, the permease and the enzyme are not produced, due to re- 

 pression by glucose. However, if the bacteria are first grown on lactose, 

 and later transferred into the medium which contains glucose and a galacto- 

 side, they will continue to make both the permease and ^-galactosidase, 

 because the permease they contain to start with is able to concentrate the 

 galactoside enough to release the glucose repression. Thus two populations 

 of identical genotype in an identical environment can be made to differ in 

 their assortment of enzymes. This situation, once it has been established, 

 is very stable ; it could be maintained for more than 150 generations (Cohn, 

 1958). 



If unadapted bacteria are given a threshold concentration of inducer for 

 a short time, the probability of acquiring the first molecule of permease is 

 not great; therefore in a population of bacteria certain individual cells will 

 adapt, and others will not. By increasing then the glucose to inducer ratio 

 in the medium adequately, it is possible to maintain in the same population 

 individual cells which do make the enzyme and others which do not. The 

 formation or non-formation of the enzyme is passed on clonally by the 

 cells to their descendants, as if it were a regular hereditary character (Cohn 

 and Horibata, 1959). Maintenance in the adapted cells is of course condi- 

 tioned by the presence of the inducer in the medium. 



This illustrates well the very complex consequences of the occurrence of 

 permeases for the regulation of enzyme synthesis. Comparable per- 

 meation systems have been found for amino acids and for various sugars and 

 derivatives. 



6. Enzymic Adaptation in Animal Tissues 



A few cases of enzymic adaptation in animals have been reported. The 

 tryptophan peroxidase activity of rat liver increases when tryptophan is 

 injected into the animal (Knox and Mehler, 1950, 1951); the same enzyme 

 can be evoked in dissociated embryonic cells (Stearns and Kostellow, 1958). 

 Tyrosine transaminase is increased by injections of tyrosine (Lin and Knox, 

 1957). A strong increase of xanthinoxidase activity in the liver of mice is 

 observed about a week after they have received injections of xanthine 

 (Dietrich, 1954). Formation of adenosine deaminase in chick embryos after 

 administration of adenosine was also reported (Gordon and Roder, 1953); 

 however, systematic studies by Solomon (1960) did not confirm this 

 observation. A strong decrease in kidney glutaminase is caused by 

 glutamate injection; this might be an example of repression (Goldstein, 

 1959). 



Enzymic adaptation in animals does not show the schematic simplicity 

 of the process found in micro-organisms. A review by Knox et al. (1956) 

 shows very well how intricate and complex is the regulation of the level of 



