140 S. S. COHEN 



amylomaltase, converting maltose to 1 mole of glucose and polymerizing the 

 residual mole of glucose to a straight chain glucose polymer, amylose. 



As in the case of xylose metabolism, there may be a number of metabolic 

 steps for which a number of enzymes may be required before insertion into 

 a constitutive path is possible. In this case, we may expect stepwise produc- 

 tion of substrates and sequential induction of each of these enzymes for these 

 substrates in turn, until a metabolite is generated wliich the constitutive 

 path is equipped to handle. Sequential induction, or "simultaneous adapta- 

 tion" has been widely used in the analysis of the paths of degradation of 

 numerous carbon compounds in organisms capable of deriving most of its 

 carbon from these substances as sole carbon sources (Stanier, 1947; Karlsson 

 and Barker, 1948; Cohen, 1949; Suda et al, 1949). The most detailed in- 

 vestigations have been those of Stanier and his collaborators (Stanier, 1955), 

 who have not only explored the utiHzation of tryptophan and many other 

 aromatic compounds in Pseudomonas by the method of sequential induction 

 but also confirmed the existence of many of the metabolic steps by demon- 

 stration of the suspected enzymes in cell-free extracts and by studying the 

 fate of isotopically labeled substrates. The method of sequential induction 

 applied to microorganisms is perhaps the simplest available for outlining a 

 suspected pathway involving penetrable intermediates and will probably 

 prove of interest in the analysis of certain pathways in animal cells as well. 

 In particular instances it has been possible to prepare inducible micro- 

 organisms as specific analytical reagents (Cohen and Raff, 1951). 



2. Problems of Protein Synthesis in Inducible Systems 



That newly induced fimctional units were actually proteins and that 

 induced biosynthesis did not involve (a) the synthesis of a coenzyme, (6) the 

 formation of special intermediates, or (c) the removal of an inhibitor were 

 clearly demonstrated by studies of extracts of galactose-induced and non- 

 induced yeast. It was shown that (a) the catalytic system for galactose 

 fermentation was not present in extracts of uninduced cells, although the 

 coenzymes required for galactose fermentation were present in such extracts, 

 and (b) the system for galactose fermentation, present in extracts of induced 

 cells, was nondialyzable and heat-sensitive, while the essential coenzymes 

 were dialyzable and heat-stable. 



The well-known case of the conversion of inactive, preformed trypsinogen 

 to enzymatically active trypsin raised the possibility of a similar occurrence 

 in all biological systems characterized by the appearance of new enzymatic 

 activities. That a comparable phenomenon, i.e., the uncovering of an inactive 

 protein or the shght remodeling of an existing protein precursor, does not 

 occur in inducible systems rested for some years on indirect evidence. In 

 addition, the situation was obscured at first by the fact that yeasts could 

 perform induced biosynthesis in the absence of an exogenous nitrogen source. 



