Regulation of Gene Action — Operons 



459 



the results obtained with this genotype are 

 summarized in Figure 36-1 along with those 

 of other hybrids which contain both the o r 

 and o+ alleles. For example, 



y + z o+ i+ /F-Lac y+ z + o c i + 



produces y + and z + enzymes but no Cz 

 substance constitutively, and it produces all 

 three in induced bacteria. Partial pheno- 

 typic analyses are available for two other 

 genotypes. Thus, 



y z + o+ i + /F-Lac y+ z o c i + 



produces Cz protein but no galactosidase in 

 noninduced bacteria, but it produces both 

 of these in induced bacteria; 



v+ z o+ i+ /F-Lac y z + o c i+ 



produces galactosidase constitutively, and 

 galactosidase and permease inductively. 



We may conclude, therefore, that these 

 results confirm the hypothesized existence 

 of an operator gene, o + , and that this gene 

 is the one sensitive to the repressor sub- 

 stance produced by the regulator gene, /+. 



When the repressor substance is produced 

 and not rendered inactive by lactose in- 

 ducer, the repressor reacts with o + ; this 

 reaction prevents both y and z alleles from 

 operating. When the mutant allele i is 

 present, no repressor is produced, o+ is 

 unaffected, and v and z alleles are capable 

 of acting constitutively. However, a mutant 

 allele of o+, namely o'\ is insensitive to the 

 repressor substance. Consequently, regard- 

 less of the genotype with respect to i, the 

 v and z alleles can act constitutively. Note 

 that the behavior of the v and z alleles de- 

 pends on which particular allele of o is in 

 the same chromosome or F particle; in other 

 words, it depends on which allele of o is 

 linked in the cis position. Thus, the con- 

 stitutive mutant of operator, o'\ has a plei- 

 otropic effect only on other genes in the 

 cis position. Recombinational studies show 

 that the locus of o r is between z and i. 



The operator gene does not seem to pro- 

 duce any unique product which can be de- 

 tected cytoplasmically. Therefore it can be 

 considered a gene whose primary job is not 



GENOTYPE 



NON-INDUCED BACTERIA INDUCED BACTERIA 



P G Cz P 



100 270 100 



100 330 100 



— 100 400 



100 300 — 



P = Permease 



G = Galactosidase 



Cz P= Cz Protein 



figure 36-1. Crosses, and their results, involv- 

 ing the Lac region of E. coli. nd = not detect- 

 able, — = not tested. 



