STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 147 



contain a fimctioning tricarboxylic acid cycle cannot handle exogenous 

 components of the cycle until after a period of exposure to these substances 

 (Karlsson and Barker, 1948; Repaske and Wilson, 1953). Davis (1956) has 

 discussed the evidence which reveals this particular case as a problem in the 

 inductive formation of a transport system. Thus, for adaptation to citrate 

 utilization in Aerobacter, net protein synthesis is required and does not occur 

 in a tryptophan-requiring mutant in the absence of the amino acid. Further- 

 more, gro^vth in the presence of glucose iidiibits the synthesis of the citrate- 

 permeabihty factors, a particularly interesting point since it has long been 

 known that glucose inhibits the induced biosynthesis of enzymes for many 

 kinds of inducers and appears to imphcate this effect in the mitial stage of 

 concentrating inducer. 



Monod and his collaborators have studied the formation of permeability 

 factors (variously called "y" factors or "permeases"^) m mduction to ^- 

 galactosides (Monod, 1956; Rickenberg et at., 1956). Using S^^-labeled methyl 

 ^-D-thiogalactoside which induced j8-galactosidase production but is not 

 hydrolyzed by the enzyme, it was found that this material could be con- 

 centrated intracellularly to 4 % of the bacterial dry weight (100 times the con- 

 centration m the medium). The concentration was energy-dependent and 

 was accompanied by an increase in oxidative metabolism (Kepes, 1957); it 

 was inhibited by dinitrophenol or azide. Addition of the inhibitor after 

 concentration released the inducer, as did other j8-galactosides. The concen- 

 trated inducer did not appear to be in a metabohcally altered state and the 

 rate of its release from the cell was a function of intracellular concentration 

 (Kepes and Monod, 1957). As noted earher, the permease systems are also 

 found m protoplasts and are therefore not cell wall components (Rickenberg, 

 1957). 



Two classes of mutants unable to use lactose as carbon sources (lac- 

 mutants) could now be distinguished. These were (1) the "absolute" mutants, 

 which are imable to produce the ^-galactosidase, but are able to develop the 

 concentrating system and (2) "cryptic" mutants, which produce normal 

 amounts of enzyme in the presence of large exogenous concentrations of the 

 methylthiogalactoside (but not lactose), but cannot develop the permease 

 for j8-galactosides, and thereby lack the abihty to handle lactose effectively. 

 The existence of these classes of mutants is thought to be a relatively general 

 phenomenon. 



The permease system must be induced before galactosidase production. 

 If glucose is normally added simultaneously with inducer, neither permease 



^ Although the name "permeases" has been given to the concentratmg systems, it is 

 not known that these systems are indeed protein catalysts. Accorduagly, it has been 

 suggested by some workers that the use of the term "permease" may be somewhat 

 premature. 



