ENZYME SUPPRESSION 69 



organism in a mixture of sugars. For example, if Esch. coli 

 is grown in a mixture of glucose and galactose, lie finds that 

 the organism utilises all the glucose before it begins to attack 

 the galactose. Galactose is attacked by means of an adaptive 

 enzyme, galactozymase, which catalyses the phosphorylation 

 to galactose-1-phosphate, and galactozymase is not formed by 

 the organism until all the glucose is removed from the medium. 

 If the organism is first grown in galactose so that it contains 

 galactozymase, and then inoculated into a mixture of glucose 

 and galactose, the galactozymase activity disappears until all 

 the glucose is again used up. In other words, the formation 

 of the constitutive glucozymase suppresses the formation of 

 the adaptive galactozymase. Monod suggests that the effect 

 is due to a definite " enzyme suppression." There is still 

 no clear explanation of the mechanism of this suppression, 

 although it can be postulated that both enzymes arise from a 

 Hmited supply of a common protein precursor : the formation 

 of the constitutive glucozymase thus uses up the available 

 precursor so that the adaptive galactozymase cannot be 

 produced. This hypothesis involves, in turn, a further 

 supposition that the active enzyme is produced by some 

 reaction between substrate and precursor, and that a substrate 

 such as glucose has a higher affinity for the precursor than a 

 substrate such as galactose. In this connection, Spiegelman 

 and his co-workers have shown, in yeast, that the formation of 

 galacozymase in washed cells is accompanied by a fall in the 

 glucozymase activity, but that if the cells are provided with 

 available nitrogen so that they can synthesise proteins without 

 drawing on their internal reserves, then the formation of the 

 new enzyme can occur without reduction in other activities. 

 All these findings emphasise that the living cell is a very 

 dynamic system with its enzymes continually undergoing 

 breakdown and resynthesis. 



Gram-positive bacteria differ from Gram-negative organisms 

 in that they are able to assimilate certain amino-acids and 

 concentrate them in the free state in the internal environment. 

 The assimilation of certain amino-acids, such as glutamic acid 



