424 



CHAPTER 46 



tutively, and to produce Cz protein also 

 inductively. This is found. The results ob- 

 tained with this genotype are summarized in 

 Figure 46-2. It is possible to construct other 

 hybrids containing both the o" and o+ alleles. 

 These are also listed in Figure 46-2 with the 

 results obtained. For example, 



y+ z 0+ i+/F-Lac y^ z+ C /+ 



produces y+ and z+ enzymes but no Cz sub- 

 stance constitutively, and produces all three 

 in induced bacteria. Partial phenotypic 

 analysis was made for two other genotypes. 

 Thus, 



;; 2+ 0+ i^lF-Lac y+ z o' i+ 



produces Cz protein but no galactosidase in 

 noninduced bacteria, but produces both of 

 these in induced bacteria. Finally, 



j+ z 0+ i^l¥-Lac y z+ o" i^ 



produces galactosidase constitutively, and it 

 and permease inductively. 



We conclude, therefore, that these results 

 are consistent with the hypothesis that an 

 operator gene, o+, exists and that it is 



this gene which is sensitive to the repressor 

 substance produced by the regulator gene, 

 /+. When the repressor substance is pro- 

 duced and is not inactivated by the presence 

 of substrate, it reacts with o+, and this pre- 

 vents both z and y alleles from operating. 

 When the mutant allele / is present, no re- 

 pressor substance is produced, 0+ is not af- 

 fected, and z and y alleles are capable of 

 acting constitutively. However, a mutant 

 allele of e»+, namely 0% is insensitive to the 

 repressor substance, so that z and y alleles 

 can act constitutively regardless of the geno- 

 type with respect to /. Note that the be- 

 havior of the y and z alleles is dependent 

 upon the particular allele of o which is in the 

 same chromosome or F particle — that is, 

 which is linked in the cis position. Thus, the 

 constitutive mutant of operator, o", has a 

 pleiotropic effect only on other genes in the 

 cis position. 



Other results demonstrate that the locus of 

 o" is between z and /. Still other mutants 

 have been obtained which prevent the syn- 

 thesis of permease and galactosidase under 

 all conditions. These mutants undergo re- 



GENOTYPE 



NON-INDUCED BACTERIA INDUCED BACTERIA 



P G Cz P 



33 36 nd 



50 no nd 



— <1 30 



nd 60 — 



P G Cz P 



100 270 100 



100 330 100 



— 100 400 



100 300 — 



P = Permease 



G = Galactosidase 



Cz P= Cz Protein 



FIGURE 46-2. Crosses, and their results, involving 

 the Lac region of E. coli. nd = not detectable, 

 — = not tested. 



