Q.35

Which of the following statement(s) is/are CORRECT for lac repressor expressed from

the lacI gene?

(A) The lac repressor is allosterically controlled and it binds to lac operator

(B) The gene lacI is in the ‘cis’ configuration with respect to lac operon

(C) The presence of glucose weakens the binding of lac repressor to lac operator

(D) The lac repressor regulates ‘in trans’ the expression of a gene cloned under the control

of lac promoter

Lac repressor, encoded by the lacI gene, is a key regulator in the lac operon of E. coli, controlling lactose metabolism through negative regulation. This SEO-optimized guide analyzes MCQ options on lac repressor function, identifying correct statements with detailed explanations for biology students and researchers.

Correct Answer

Statements (A) and (D) are correct. The lac repressor is allosterically regulated by lactose (via allolactose) and acts in trans on lac promoter-controlled genes, while options (B) and (C) are incorrect due to cis configuration of lacI and indirect glucose effects via catabolite repression.

Option A Analysis

The lac repressor binds to the lac operator to block transcription and undergoes allosteric conformational change upon binding allolactose (lactose derivative), releasing from the operator to allow expression. This classic allosteric control exemplifies negative inducible regulation in the lac operon.

Option B Analysis

The lacI gene is located adjacent to the lac operon but produces a diffusible repressor protein that regulates operators on the same DNA molecule (cis) or other copies, yet the statement claims lacI is strictly “in cis” configuration, which misrepresents its trans-acting nature despite physical proximity.

Option C Analysis

Glucose does not directly weaken lac repressor-operator binding; instead, high glucose lowers cAMP levels, preventing CAP-cAMP activation of the lac promoter (catabolite repression), while repressor binding remains lactose-dependent.

Option D Analysis

Lac repressor diffuses freely and regulates genes in trans, meaning lacI from one chromosome can repress lac operators on another chromosome or plasmids with lac promoter, as demonstrated in classic merodiploid experiments.

Lac Operon Regulation Overview

• Repressor Action: Tetrameric lac repressor binds operator in absence of inducer, preventing RNA polymerase access.

• Induction: Allolactose binds repressor, causing release (allosteric effect).

• Catabolite Repression: Low glucose enables cAMP-CAP binding for full activation; independent of repressor.
  This system ensures efficient lactose utilization only when needed.