8. In a lac operon, a nonsense mutation in the gene encoding beta-galactosidase was found to interfere with the expression of downstream permease and transacetylase genes. Which one of the following may explain this observation most appropriately?
(1) Polar effect of the mutation
(2) trans-effect of the mutation
(3) Binding of the release factor to the nonsense codon prevents translation of the downstream cistrons
(4) Formation of a stem-loop structure in the upstream cistron prevents translation of downstream cistrons.
In the lac operon, a nonsense mutation in the lacZ gene encoding beta-galactosidase disrupts expression of downstream lacY (permease) and lacA (transacetylase) genes due to the polar effect, where premature translation termination reduces ribosome protection on polycistronic mRNA, exposing it to Rho-dependent termination or degradation.
Lac Operon Basics
The lac operon in E. coli features polycistronic mRNA transcribing lacZ (beta-galactosidase for lactose hydrolysis), lacY (permease for lactose uptake), and lacA (transacetylase for byproduct detoxification) sequentially. Nonsense mutations introduce premature stop codons (UAA, UAG, UGA), halting translation early.
Option Analysis
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(1) Polar effect of the mutation: Correct. Premature ribosome release from lacZ mRNA lowers downstream ribosome density, allowing Rho factor access for transcription termination or mRNA decay, suppressing lacY and lacA.
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(2) trans-effect of the mutation: Incorrect. Trans-effects involve diffusible regulators like repressors affecting multiple operon copies; this mutation acts cis on the same mRNA.
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(3) Binding of the release factor to the nonsense codon prevents translation of the downstream cistrons: Incorrect. Release factors (RF1/RF2) terminate upstream translation but do not block downstream initiation; polarity arises from secondary effects like mRNA instability.
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(4) Formation of a stem-loop structure in the upstream cistron prevents translation of downstream cistrons: Incorrect. Stem-loops cause Rho-independent transcription termination, unrelated to translation nonsense codons.
Polar mutations are strongest near inter-cistronic regions, decreasing with distance from nonsense codon to downstream genes. This CSIR NET concept highlights prokaryotic gene regulation coupling transcription and translation.
