• Riboswitches in Gene Regulation:
• Riboswitches are structured cis-regulatory RNA elements located predominantly within the $5'$ untranslated region (5' UTR) of bacterial mRNAs.
• Two Functional Domains:

• Aptamer Domain: Directly and selectively binds a specific small metabolite (e.g., TPP, FMN, SAM, lysine, guanine) without requiring protein cofactors.
• Expression Platform: Undergoes conformational structural rearrangement upon metabolite binding, forming alternative terminator/antiterminator hairpins that modulate downstream transcription elongation or translation initiation.
  • Exam Context: Option (4) was designated in the official answer key describing the metabolite-induced conformational change governing downstream gene expression.

• Eukaryotic RNA Polymerases and Their Transcripts:
• RNA Polymerase I: Synthesizes the large $45\text{S}$ pre-rRNA precursor in the nucleolus, which is processed into $28\text{S}$, $18\text{S}$, and $5.8\text{S}$ rRNAs.
• RNA Polymerase II: Synthesizes all messenger RNAs (mRNAs) and most snRNAs, microRNAs, and lncRNAs.
• RNA Polymerase III: Transcribes small structural RNAs, specifically transfer RNAs (tRNAs), $5\text{S}$ ribosomal RNA, and the U6 spliceosomal snRNA.
• RNA Polymerase IV / V: Plant-specific polymerases involved in RNA-directed DNA methylation.

Correct option: (3) RNA polymerase Ill

• Catabolite Repression of the lac Operon:
• In E. coli, when both glucose and lactose are available, the bacterium utilizes glucose preferentially via catabolite repression (producing diauxic growth):
• Glucose uptake through the phosphotransferase system (PTS) inhibits adenylyl cyclase, keeping intracellular cAMP levels very low.
• In the absence of cAMP, the Catabolite Activator Protein (CAP / CRP) cannot bind to the lac promoter to recruit RNA polymerase.
• Thus, transcription of the lac operon remains repressed until glucose is completely exhausted, even though lactose is present to bind the LacI repressor.

Correct option: (2) The lac operon is repressed due to glucose presence, and lactose metabolism occurs only when glucose is depleted.