You wish to isolate lac mutants of E. coli. After mutagenesis, mutants that DO NOT express any beta-gal in inducing medium, are candidates for loss of function at: (choose the answer with all and only the pertinent loci)

Note. For this answer, let's use the recent and strict book definition of P (binds RNA pol) and O (binds lacI product) sites, not the encompassing definition originally used originally by Jacob and Monod.

a) O, Y

b) I, O

c) Z, O

d) A, Y, O

e) Z, P

The correct option is e.) Z,P

Step-by-step explanation

1. The correct option is e. Z,P

Lac operon is a cluster of genes under the control of a single promoter responsible for metabolism of lactose in E.coli.

The following genes are present in the Lac operon:

1. P (promoter): region where RNA polymerase binds

2. O (operator): region where inducer binds and prevents movement of RNA polymerase

3. LacZ: codes for the enzyme beta-galactosidase that breaks down lactose to glucose and galactose

4. LacY: codes for permease that allows movement of lactose inside the bacterial cell

5. LacA: codes for transacetylase that transfers acetyl group to beta-galactosides.

6. LacI: codes for the lac inducer which binds to operator to prevent movement of RNA polymerase.

If we want to isolate a mutant that does not express beta-galactosidase then the mutant should have a mutated lacZ site because mutation in this region would prevent the expression of enzyme causing loss of function. Also, the promoter is essential for transcription of lac operon because it allows binding of RNA polymerase which then synthesizes the mRNA from the Lac Z, Y and A. Therefore, a mutation in P will prevent transcription of beta-galactosidase mRNA as well causing a loss of function.   

2. The option a is incorrect because mutating the Y gene wouldn't affect the expression of beta-galactosidase because it codes for permease. Also, mutating the operator would prevent the binding of inducer to operator thereby causing a gain of function which leads to constant expression of beta-galactosidase.

3. The option b is incorrect because mutating the I gene would prevent formation of inducer leading to constant expression of beta-galactosidase.

4. The option c is incorrect because mutating the operator would prevent the binding of inducer to operator thereby causing a gain of function which leads to constant expression of beta-galactosidase.

5. The option d is incorrect because mutating the Lac A gene would prevent formation of acetylase and wouldn't have any effect on expression of beta-galactosidase. Also, mutating the Y gene wouldn't affect the expression of beta-galactosidase because it codes for permease.