Two other proteins are also induced: Galactoside permease and thiogalactoside transacetylase (lactose transport) (detoxification) These proteins are associated with the same environmental change: Expression levels are regulated together! Such coordinated unit of gene expression: “Operon”
-galactosidase : z galactoside permease : y thiogalactoside transacetylase : a Mutant studies: z - y + a + no z; y and a are ok Mutant: z, y, a are expressed in the absence of lactose constitutive mutant
Mutant studies: Mutant: z, y, a are expressed in the absence of lactose constitutive mutant All three genes are regulated by a common element that is different from z, y, a themselves: “ i ” Wildtype inducible: i + z + y + a + Constitutive mutant: i - z + y + a + Is i activator or repressor?
Mutant studies: Episome: genetic material (plasmid) Diploid bacteria (by conjunction) Example: i + z - / (e)i - z + (chromosome/episome) Is this strain inducible or constitutive for -gal? Does i on the chromosome repress z on episome? How about i - z + / (e)i + z - ? A diffusible repressor is encoded by the i gene trans-acting factor
The operon model p: promoter sites o: operator site (regulatory DNA seq.) i: the repressor, binds to o z;y;a : polygenic or polycistronic RNA transcript
Another example: pur repressor and pur operon pur repressor: * dimeric, 31% identical to lac repressor, similar in 3D structure * represses genes involved in purine biosynthesis * pur repressor binds DNA only when bound to a small molecule corepressor * Binds specific inverted-repeated sequence