Questions: Transcription Elongation and Termination

Question 1

A mutation eliminates the GC-rich palindrome in a prokaryotic rho-independent termination sequence but leaves the downstream polyU run intact. What would you predict about termination at this site?

• A. Termination is unaffected — the polyU run alone is sufficient to release the polymerase
• B. Termination is significantly impaired — both the hairpin structure and the weak rU-dA hybrid cooperate to destabilize the complex; removing the hairpin eliminates a critical component
• C. Rho-dependent termination will automatically compensate at all sites lacking a hairpin
• D. The polyU run becomes more effective at termination without the competing hairpin structure

Answer: (B)

Rho-independent termination depends on two cooperating features: (1) the GC-rich hairpin, which physically tugs at the polymerase exit channel, and (2) the weak rU-dA base pairs at the RNA-DNA hybrid, which provide minimal stabilization. Neither alone is sufficient for efficient termination — it is the combination of mechanical destabilization from the hairpin plus the inherent fragility of the rU-dA hybrid that reliably releases the polymerase. Eliminating the hairpin leaves only the weak hybrid, which is usually insufficient.

Question 2

Eukaryotic RNA Pol II transcription termination is mechanistically distinct from prokaryotic termination primarily because:

• A. Eukaryotic RNA polymerase is too large to form stable hairpin structures in the exit channel
• B. Eukaryotic termination is coupled to 3' end cleavage and polyadenylation — CPSF recognizes the AAUAAA signal, cleaves the transcript, and a torpedo exonuclease dislodges the downstream polymerase
• C. Eukaryotic cells lack Rho protein homologs, forcing them to use a different helicase mechanism
• D. Eukaryotic termination uses the same GC-rich hairpin mechanism but adds a poly(A) tail rather than releasing the polymerase directly

Answer: (B)

Eukaryotic Pol II termination is intimately linked to 3' processing. The CPSF complex recognizes the AAUAAA polyadenylation signal, cleaves the transcript ~10-30 nt downstream, and hands the cut 3' end to poly(A) polymerase. The polymerase continues briefly past the cleavage site, but a torpedo exonuclease degrades the remaining RNA tether from the 5' end, ultimately dislodging the polymerase. This mechanism ensures every mature mRNA receives a poly(A) tail as part of the same coupled reaction.

Question 3

In rho-independent termination, both the GC-rich RNA hairpin and the weak rU-dA base pairs at the RNA-DNA hybrid are required — they cooperate to release the polymerase.

• T. True
• F. False

Answer: True

The two features work together: the hairpin forms immediately at the polymerase exit channel and exerts a physical destabilizing force, while the rU:dA base pairs (the weakest in nucleic acid chemistry) provide minimal resistance to dissociation. Experimental deletion of either element reduces termination efficiency. The combination of active mechanical disruption (hairpin) and passive instability (weak hybrid) creates a reliable termination mechanism encoded entirely in the DNA/RNA sequence.

Question 4

The Rho protein terminates transcription by recognizing a specific DNA sequence and physically blocking RNA polymerase from advancing past that point.

• T. True
• F. False

Answer: False

Rho acts on RNA, not DNA. It loads onto a specific unstructured region of the nascent RNA called the rut site (rho utilization site) and translocates along the RNA in the 5' to 3' direction as an ATP-dependent helicase, chasing the polymerase. When the polymerase pauses at a downstream site, Rho catches up and uses its helicase activity to unwind the RNA-DNA hybrid, releasing the transcript. Rho-dependent termination is a kinetic race between RNA synthesis and Rho translocation.

Question 5

Why is it significant that eukaryotic transcription termination is coupled to polyadenylation rather than occurring as an independent event?

Model answer: Coupling ensures that every mRNA receives its poly(A) tail before release — the CPSF cleavage generates the 3' end that poly(A) polymerase then extends. This linkage prevents the release of untailed mRNAs, which would be rapidly degraded and poorly exported from the nucleus. It also provides a precise, regulated endpoint: termination only occurs at AAUAAA-containing sequences, not at arbitrary positions, so the 3' end of each transcript is defined by its processing signal rather than by a separate termination element.

The coupling of termination to 3' processing is a eukaryotic innovation that integrates two steps that are separate in prokaryotes. In bacteria, transcription termination is independent of any downstream processing. In eukaryotes, the polyadenylation signal simultaneously triggers cleavage (creating the mRNA 3' end), poly(A) tail addition (stabilizing the transcript), and eventual polymerase release (via torpedo mechanism). This tight linkage means that efficient termination and proper 3' end formation are inseparable.