Rho-dependent termination in E. coli: Which statements about the mechanism are correct?
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AIt requires ATP hydrolysis by Rho factor
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BIt needs a stretch (about 50 nt) of unstructured, C-rich mRNA for Rho loading
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CBoth (a) and (b)
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DIt passively stops transcription without releasing mRNA or the polymerase
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EIt occurs only at promoters with strong −10 elements
Answer
Correct Answer: Both (a) and (b)
Explanation
Introduction / Context: Termination of transcription in bacteria can be intrinsic (Rho-independent) or Rho-dependent. Knowing the requirements for Rho-dependent termination clarifies how certain transcripts are selectively terminated.
Given Data / Assumptions:
- Rho is an RNA-dependent ATPase/helicase.
- Rho binds specific RNA regions (rut sites) that are typically C-rich and unstructured.
Concept / Approach: Rho loads onto the nascent RNA at a rut site and translocates 5'→3' along the RNA using ATP. When RNA polymerase pauses downstream, Rho catches up and promotes dissociation of the transcription complex, releasing RNA.
Step-by-Step Solution: Confirm ATP requirement → Rho translocation consumes ATP. Confirm RNA features → rut sites often ~50 nt, C-rich, and relatively unstructured to allow Rho loading. Select the combined answer acknowledging both requirements.
Verification / Alternative check: Mutations that disrupt rut sites or ATPase activity of Rho impair termination; transcriptional pausing elements enhance Rho catch-up.
Why Other Options Are Wrong: Option d contradicts the release function; option e confuses promoter strength with termination determinants.
Common Pitfalls: Mixing intrinsic terminators (hairpin + poly-U) with Rho-dependent mechanisms; assuming Rho acts without energy.
Final Answer: Both (a) and (b).