Physiological Mechanisms of Resistance 175 



permeation systems has been the demonstration that certain 

 of these systems are adaptive, i.e. they appear only when the 

 cells are grown in the presence of the substrate or a related 

 inducer. This adaptability has been demonstrated for the 

 citrate system in Pseudomonas (Barrett, Larson and Kallio, 

 1953; Kogut and Podoski, 1953) and Aerobacter (Green, 

 1956), and for the p-galactoside transport system in Esch. coli 

 (Davis, 1956; Monod, 1956). The systems resemble adaptive 

 (inducible) enzymes in two further respects: the adaptation 

 requires conditions that permit protein synthesis; and it is 

 blocked by the presence of glucose or other carbohydrates 

 which are known, in contrast to lactate or succinate, to block 

 formation of many adaptive enzymes (Green, 1956; Davis, 

 1956; Monod, 1956; Rickenberg et ah, 1956). 



The kinetics of the formation and action of an adaptive 

 permeation system have been elegantly analysed by Cohen, 

 Monod, and co-workers (Monod, 1956; Rickenberg et ah, 1956), 

 employing p-galactosides. This system has the advantage 

 that a substance is available, p-thiomethyl galactoside 

 (TMG), which induces and is transported by the permeation 

 system but is not metabolized; hence it is possible to study the 

 ability of the system not only to transport but also to concen- 

 trate substrate. It would be inappropriate to review here all 

 this work; but two further properties of the permeation 

 systems should be noted. They resemble enzymes in their 

 kinetics, which can be analysed in terms of a Michaelis 

 constant, and they exhibit typical competition between 

 structural analogues. Finally, the same group have also 

 studied in detail analogous systems for concentrating various 

 amino acids (Cohen and Rickenberg, 1956). These systems 

 resemble the one for p-galactosides except that they appear to 

 be constitutive rather than adaptive.* 



* Monod has referred to stereospecific permeation systems as permeases 

 (Rickenberg et al., 1956), a term which seems to imply an enzymic nature. It 

 seems preferable to avoid such a mechanistic term at this time, since future 

 work will have to determine whether or not the action of these systems involves 

 enzymic conversion of the substrate to another compound in the course of 

 transport. 



