chairman's introduction 577 



substrate (or an analogue), and can be repressed by the presence of a 

 preferred foodstuff such as glucose. Induction of citrate transport in 

 Pseudomonas [ii, 12], and induction and repression of citrate transport in 

 Aerobacter [13] and ^-galactoside transport in Escherichia coli [7, 14], were 

 discovered independently in four laboratories. It is of interest to note, in 

 all these early communications, reluctance to trust the conclusion, however 

 logically derived from the evidence, that the properties of a cell membrane 

 could be substantially modified by the nature of the growth medium. 

 Apparently such a flighty disposition, responsive to suggestions from the 

 environment, was easier to ascribe to invisible molecules in the cytoplasm 

 than to a solid, microscopically visible structure like a membrane ! 



These, then, are the major kinds of evidence available for the existence 

 of specific transport systems. The control of these systems that is possible 

 in rnicrobes has, of course, opened up new avenues of approach to their 

 nature. One of the most significant findings has been that the formation of 

 the inducible transport systems for citrate [13] and /3-galactosides [7, 8], 

 like the formation of inducible enzymes, requires conditions that permit 

 protein synthesis. We thus have strong evidence for a proposal offered by 

 earlier permeability workers on more speculative grounds (cf. [15, 16]): 

 that the specificity of transport systems must depend on the presence of 

 proteins with a specificity similar to that already familiar in enzymes. It 

 has also been possible, by varying the number of "pumps" per cell 

 (through partial induction), to analyze more deeply the kinetics of entry 

 and exit in a system carrying out active transport. 



Monod has proposed the term "permease" for specific transport 

 systems, whether active or passive [7, 8]. This term has the advantage of 

 focusing attention on a most important property of these systems: the 

 presence of elements which resemble enzymes in their specificity, in their 

 mass-law relation to substrate, and in the genetic and environmental 

 factors influencing their formation. The term, however, has serious 

 disadvantages. First, being based on the historical discontinuity between 

 studies on bacteria and those on other cells, it has been construed as 

 implying entities quite different from the "carriers" proposed by Danielli, 

 Le Fevre, Widdas, Wilbrandt, and Ussing (cf. [17]) to account for specific 

 transport in animal cells. In fact, however, the /z<//r//o//rt/ properties of the 

 two systems are essentially indistinguishable ; the novel feature of the 

 bacterial systems has been the possibility of controlling their formation. 

 Until proved otherwise, it would seem wiser to assume that the specific 

 transport systems of all cells are fundamentally similar ; a unified termin- 

 ology would thus be desirable. And while "permease" has been widely 

 used with reference to bacteria, it does not seem to have been extended to 

 other cells. 



VOL. II. — 2 P 



