Question

In: Biology

For each of the E. coli strains below, indicate whether β-galactosidase and permease expression are inducible,...

For each of the E. coli strains below, indicate whether β-galactosidase and permease expression are inducible, constitutive, or not expressed. Assume that no glucose is available to the cells. β-galactosidase Permease I+ P— O+ Z+ Y— I+ P+ Oc Z+ Y— I+ P+ OC Z+ Y— /I—P+ O+ Z— Y+ IS P+ OC Z— Y+ /I+P+ O+ Z+ Y—

Solutions

Expert Solution

Answer- The lactose operon (lac operon) is an operon required for the transport and metabolism of lactose in E.coli

The lac operon consists of 3 structural genes, and a promoter, a terminator, regulator, and an operator. The three structural genes are: lacZ, lacY, and lacA.

lacZ encodes β-galactosidase (LacZ), an intracellular enzyme that cleaves the the disaccharide lactose into glucose and galactose.

lacY encodes Beta-galactoside permease (LacY) which increases the permeability of the cell to β-galactosides.

lacA encodes β-galactoside transacetylase (LacA), an enzyme that transfers an acetyl group from acetyl-CoA to β-galactosides.

The promoter is the binding site for RNA polymerase, the enzyme that performs transcription.

The operator is a negative regulatory site bound by the lac repressor protein.

The operator overlaps with the promoter, and when the lac repressor is bound, RNA polymerase cannot bind to the promoter and start transcription.

Mutations in lac operon-

repressor mutants- repressor is non functional:  cannot bind Operator no repression, operon is constitutively active

LacIs = “super-repressor”  can bind operator even when lactose is bound. Operon is constitutively inactive.

Oc operator is defective, no longer binds repressor

lac Z- mutant: no functional β-galactosidase protein

lac Y- mutant no functional permease.

Now the genotypes given in question

1) I+P-O+Z+Y- here mutation is in promoter region, hence RNA polymerase will not bind over it and no transcription takes place, so here  β-galactosidase and permease will not express.

2) I+P+OcZ+Y-    here operator and permease (Y) gene is mutant, operator will not  binds repressor and Operon is constitutively active so β-galactosidase is constitutively expressed and permease is not expressed

3) I+P+OcZ+Y-  / I- P+ O+ Z- Y+ here in one operator and permease (Y) gene is mutant and in second repressor and  β-galactosidase gene are mutant. So expression of β-galactosidase is constitutive  and permease is inducible because repressor is transacting so repressor produced from first will bind over the operator region of second and make it inducible.

4)   Is P+OcZ- Y+ / I+ P+ O+ Z+ Y- here in first repressor and operator are mutant so repressor will not bind over operator and permease will be constitutively expressed, in second operator is not mutant but repressor is transacting so repressor from first will bind over it make it constitutively inactive. so β-galactosidase will not express

β-galactosidase permease
I+P-O+Z+Y- no expression no expression
I+P+OcZ+Y-    constitutive no expression
I+P+OcZ+Y-  / I- P+ O+ Z- Y+ constitutive inducible
Is P+OcZ- Y+ / I+ P+ O+ Z+ Y- no expression constitutive

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