128 THE BIOSYNTHESIS OF PROTEINS 



the amount of enzyme may depend on the inducer. One can induce the 

 synthesis of another enzyme, galactokinase, in E. coli in the absence of any 

 exogeneous inducer by causing the development of a temperate phage which 

 had introduced the corresponding structural gene by transduction (Buttin 

 eifl/., 1960). 



Mutants which normally produce the enzyme in the absence of any 

 added inducers have been isolated in a few cases; these are 'constitutive' 

 mutants. Penicillinase produced by a constitutive mutant of B, cereus was 

 compared to the induced enzyme made by the adaptive parent strain. None 

 of the physical, chemical, enzymological and serological tests applied could 

 reveal any difference between the constitutive and the induced enzymes, 

 which had both been highly purified and crystallized (Kogut et ah, 1956; 

 Pollock, 1956). Similar, although less complete evidence for identity of 

 constitutive and inducible enzyme were obtained for the penicillinase of 

 B. suhtilis (Manson et ah, 1954) and for the j8-galactosidaseof £". co//(Monod 

 and Cohn, 1951) and oi Neurospora crassa (Landman, 1954). 



The capacity of constitutive mutants to produce perfect enzyme in the 

 absence of added inducer raises an interesting dilemma: are these strains 

 constitutive because they can really make the enzyme without any parti- 

 cipation of an inducer, or is it because they themselves manufacture the 

 inducer, whereas the adaptive strains cannot do so? The second alternative 

 at first looked very promising: it provided a unitary view (Cohn and Monod, 

 1956) in which all the enzymes were regarded as inducible, the constitutive 

 enzymes being induced by an endogeneous inducer. Moreover, the 

 occurrence of endogeneous inducers was indicated by numerous cases of 

 sequential induction (Stanier, 1947, 1950, 1955; Suda et ah, 1949). 



When the synthesis of an enzyme is induced by the substrate, the inducer 

 substrate is acted upon by the induced enzyme and it often happens that 

 the product of this enzymic transformation induces the formation of a 

 second enzyme. Addition of the first inducer in such a case results in the 

 sequential synthesis of two enzymes. The second one is actually induced 

 by an endogeneous inducer. If this process repeats itself several times, a 

 series of enzymes can be induced in succession. The classical example is 

 the sequential induction in P. fluorescens of seven enzymes which catalyse 

 the oxidation of the indol ring of tryptophan into ^-keto-adipic acid in seven 

 successive steps. It is assumed that each intermediary metabolite acts as an 

 inducer for the synthesis of the next enzyme. It is remarkable that in many 

 cases the different enzymes are actually made in succession. Kinetic studies 

 on the utilization of intermediates or on the formation of enzymes made it 

 possible to unravel several metabolic pathways by making use of sequential 

 induction (Stanier, 1947; Karlson and Barker, 1948; Ajl, 1950; Wiame, 

 1951). 



If all the enzymes were induced either by an endogeneous or by an 



