ENZYME INDUCTION 87 



been reviewed several times in the past 40 years (4, 37, MO, 245). 

 Amylase, inulase, maltase, sucrase, lipase, pectin-polygalacturonase, 

 and tannase are among the more frequently investigated enzymes. 

 The results with pectin-polygalacturonase are typical, in that sub- 

 stances chemically related to the substrate may also increase enzyme 

 formation (156), and in that the response to substrate occurs with 

 some fungi and not with others (25). Here again, it must be stressed 

 that a genetic mechanism — selection of variant cells or of variant nuclei 

 in a heterocaryon — is as probable in many instances as a physiological 

 mechanism. 



Enzyme induction has been defined (42) as a relative increase in the 

 rate of synthesis of a specific apoenzyme occurring in response to a 

 specific chemical substance, the inducer. Earlier, attention was focused 

 on the specific substrate as inducer, and the term "adaptive enzyme" 

 was employed to denote an enzyme formed only or in increased 

 amounts in the presence of substrate; constitutive enzymes are defined 

 as those which are formed in quantity in a favorable medium lacking 

 substrate or inducer (103). The inducer is most often also the sub- 

 strate, but it may be a chemically related or an unrelated compound. 

 In a classical experiment (196), resting cells of Escherichia coli form 

 the enzyme formic dehydrogenase if incubated in the presence of 

 formate, not otherwise. 



It has been shown by Landman (111) that the lactase (/3-galactosi- 

 dase) of Neurospora crassa is induced under conditions which satisfy 

 the requirements above, particularly in that lactase is formed by 

 resting cells exposed to an inducer (Figure 3). The data show also 

 that sugars other than lactose act as inducers. Resting cells respond 

 somewhat differently from growing cells, and the mechanisms may not 

 be identical. The time required for lactase induction in N. crassa is 

 much longer than that required for induction of the same enzyme in 

 yeast or in Escherichia coli. 



The foregoing demonstration is the only one to date in the fungi. 

 It seems reasonable, however, to postulate, in agreement with Monod 

 (132), that at least some of the reports of adaptive growth and of stimu- 

 lation of enzyme formation by substrate which have been mentioned 

 do in fact involve enzyme induction, even though genetic change is not 

 rigorously excluded. 



Earlier in this chapter, particularly in reference to the growth of 

 fungi with fructose and mannose, it was suggested that results with 

 mixed carbon sources may be explained in part by the induction of 

 new enzymes. Specifically, the effect of a small amount of glucose in 

 permitting rapid growth on fructose or mannose, sugars which by 



