336 CELL HEREDITY 



TABLE 11.3 

 Classes of Mutants at the lac Locus in f . co/i 



i ~ y'^ wild type; inducible formation of /3-galacto.sidase and galactoside- 

 permease 



z y constitutive mutant; /3-gal. and gal. -permease always present 



- y~ cryptic mutant; i3-gal. inducible, permease never present 



y~ cryptic-constitutive; i3-gal. always present, permease never present 



y /3-giil. never present, permease inducible 



~ y i3-gal. never present, permease always present 



~ y~ neither i8-gal. or permease is ever found 



z y neither jS-gal. or permease is ever found 



o^ z'^ y^ partially constitutive when o^ is in cis position with other genes 



+ _- 



- _- . + 



o° z y neither i3-gal. or permease is inducible (mimics deletion) 



been described, which vary in their basal enzyme content and response 

 to inducers; they are listed in Table 11.3. The letter z refers to the 

 genetic capacity of the strain to make /3-galactosidase, y refers to galac- 

 toside-permease, and i refers to inducibilitv. It is evident that here, as 

 in the penicillinase system, the ability to make the enzyme is separable 

 from its inducibility. 



The genetic significance of so-called constitutive mutants can hardly 

 be overstated, for they have provided the first direct evidence that the 

 property of inducibility-repressibility is under genetic control. The 

 model scheme outlined above included a parameter for gene control, in 

 the R-moiety of the repressor, but did not specify how the gene might, 

 in turn, be regulated. The fact that mutants arise that manufacture 

 enzyme at a constant, inflexible rate shows that inducers and repressors 

 are not essential elements of the enzyme-forming system, but rather that 

 they are dispensable regulators. We may now formulate a new question: 

 What is the nature of the constitutive mutation, and what light does it 

 cast upon genetic mechanisms of regulation? 



In tests both by recombination and by transduction, the y, z, and i 

 factors have been found to be closely linked in the so-called "lac" region 

 of the E. coli chromosome. In a study of interactions among these 

 factors, crosses were made of various mutants and measurements of 

 /3-galact()sidase were performed on the resulting /Agotes. The results 

 given in Figure 11.16, were obtained with zygotes from the cross: 



Hfrz^ t"*" Str-s T6-s x F~ z' i~ Str-r Tb-r (cross 1 ) 



When the Hfr chromosome, carrying z'^ i'^ enters the F~ cell which is 

 z~ r, /3-gaIactosidase formation begins at once, in the absence of added 



