In bacteria, which termination mechanism uses a GC-rich hairpin followed by a poly-U tail?

• A. It requires a rho factor to terminate transcription
• B. It uses a GC-rich hairpin followed by a poly-U tail to destabilize the RNA-DNA hybrid
• C. It occurs only in eukaryotes
• D. It ends with transcription of tRNA

Answer: (B)

This question tests intrinsic (Rho-independent) termination in bacteria. In this mechanism, the newly made RNA forms a GC-rich hairpin structure (a stem-loop) followed directly by a sequence of uracils. The hairpin is created by inverted repeats in the DNA that fold the nascent RNA into a stable stem formed mainly by G-C base pairs. After the hairpin, the RNA has a run of uracils, which pair weakly with the DNA template (A-U pairs are less stable than G-C). This combination causes the RNA polymerase to pause at the hairpin and then dissociate as the RNA-DNA hybrid becomes unstable, releasing the transcript.

This is different from rho-dependent termination, which requires the rho protein to disrupt the transcription complex. It’s a bacterial mechanism, not exclusive to eukaryotes, and it isn’t about terminating transcription of a tRNA gene specifically.