POPULATION GENETICS 



earlier work of Monod and Gohn (1952) on the induction 

 of the adaptive enzyme /^-galactosidase by an inducer, thio- 

 methyl y^-D-galactoside, TMG, which is not a substrate for 

 the enzyme. There are two reasons for bringing these 

 observations into this account of clonal behaviour. The first 

 is to draw attention to the modifications that have taken 

 place in the interpretation of adaptive enzyme formation in 

 recent years. The simple belief that the bacterial cell con- 

 fronted with a new substrate forthwith produced a new 

 enzyme to deal with it has vanished, just as, I believe, the 

 simple picture of antibody as a protein created or modified 

 on the spot to deal with a new antigen will also be discarded. 

 The second reason is the demonstration in this work of 

 Novick and Weiner of a process which has superficially all 

 the marks of a mutation but which can be shown to be a non- 

 genetic physiological change. 



TMG and other inducers of y^-galactosidase have two 

 actions on E. coli. Both appear to involve enzyme activation. 

 The first is to induce a specific permeability of the cell wall, 

 ascribed to the activation of a 'permease'. This allows the 

 concentration of TMG inside the cell to a level which may be 

 100 times what it is in the medium. It is this intracellular 

 concentration which determines the activation of the cell 

 to produce and liberate /?-galactosidase. 



At low concentrations of inducer the primary induction 

 of permease is slow, but, wherever this is accomplished, the 

 bacterium involved takes in a relatively large amount of 

 inducer which can now maintain the production of permease 

 and induce y^-galactosidase production both in itself and in 

 its descendants. Bacteria which have not yet been rendered 

 permeable are not induced. Such a population is composed 

 of two sorts of bacterium : fully induced and not induced. 

 This can be shown by choosing a ' maintenance concentration ' 

 of TMG (5xio~^m) which for pre-induced bacteria will 

 allow full synthesis of adaptive enzyme but has only a 

 negligibly small power to induce normal bacteria. If limit- 



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