576 BERNARD D. DAVIS 



which provided many examples of apparent intermediates that could not 

 serve as a growth factor, presumably because of a permeability barrier. In 

 at least two cases, involving mutants blocked before 5-dehydroquinate [10] 

 and citrate [2], it was possible to select secondary mutants that had gained 

 the ability to grow on the intermediate. Since the required enzymes were 

 already present in the cell before this second mutation, it seemed evident 

 that a one-step mutation had altered the permeability properties of the cell. 



A much more extensive exploration of mutational effects on a transport 

 system has been provided by Cohen and Monod for the /3-galactoside 

 system [8]. It has been demonstrated that the gene controlling the forma- 

 tion of the transport system and that controlling formation of the enzyme 

 ^-galactosidase are distinct though closely linked on the chromosome: a 

 mutation can prevent or restore the formation of either without affecting 

 the other. Of particular interest, for present purposes, are two properties 

 of a cryptic mutant, i.e. one which retains /S-galactosidase but is transport- 

 negative for ^-galactosides : 



(i) Compared with a transport-positive, the transport-negative strain 

 metabolized /S-galactosides very much more slowly; and the relation of 

 rate to substrate concentration implied that the rate-limiting step in this 

 strain is diffusion rather than the action of a system characterized by a 

 Michaelis constant. We thus see that in the absence of a specific transport 

 system the same permeant can penetrate slowly, presumably via a more 

 primitive mechanism. 



(2) In addition, the transport-negative strain had lost not only the 

 capacity to metabolize lactose rapidly but also the capacity to concentrate a 

 non-metabolized ^-galactoside (TMG). This finding is important in 

 linking studies on active concentration and those on rate of substrate 

 utilization to the same functional unit. F^or the loss of apparently active 

 concentration could conceivably also be due to loss of ability to convert 

 the permeant into a labile intracellular derivative, and loss of rapid 

 utilization could conceivably be due to formation of the intracellular 

 enzyme in a masked form; but only loss of a specific transport system 

 could singly account for both effects of the mutation. 



Incidentally, metabolic inhibitors such as azide eliminated the active 

 concentration but not the rapid utilization, suggesting that the same 

 specific transport system, which requires an energy supply for function in 

 active transport, may in the absence of an energy supply still function in 

 passive transport [8]. 



6. Induction and repression 



Another novel contribution from bacteria was the finding that in an 

 appropriate cell the presence of certain transport systems, like that of 

 certain enzymes, requires induction by growth in the presence of the 



