Rho-dependent and intrinsic termination
/ ROH-dependent /
Bacteria need two different ways to stop a runaway copyist, and they are nicely contrasting: one is built into the message itself, the other relies on a dedicated chaser protein. Together they show that 'how to stop' can be solved either by clever sequence design or by a hardworking helper.
Intrinsic (factor-independent) termination is the self-contained method. Near the end of the gene, the transcribed RNA contains a stretch that is self-complementary, so the freshly made RNA folds back on itself into a stable hairpin loop; immediately after the hairpin comes a run of uracils. The hairpin tugs at the polymerase and stalls it, and because U-A base pairs are the weakest, the RNA in the RNA-DNA hybrid lets go easily — the transcript pops free and the polymerase falls off, no extra protein required. Rho-dependent termination uses a protein instead: Rho is a ring-shaped enzyme that grabs onto a specific stretch of the RNA behind the polymerase and, burning ATP, motors along the transcript toward the enzyme. When it catches a paused polymerase, it pries the RNA-DNA hybrid apart and releases everything. Picture intrinsic termination as a built-in tripwire and Rho as a guard who runs down the track and tackles the runner.
Which mechanism a gene uses affects how it can be regulated, and Rho turns out to be essential and a target: blocking Rho is lethal to many bacteria, which is being explored for antibiotics. The contrast also previews a broader theme — termination is not just an off-switch but a decision point. In attenuation, for example, whether an intrinsic terminator forms early or is bypassed determines how much of certain genes is transcribed, letting bacteria respond to conditions while transcription is still in progress.
Intrinsic: ...GCGCC-hairpin-loop-GGCGC followed by UUUUUUU, all in the RNA itself. Rho-dependent: no such hairpin; instead Rho loads onto a C-rich RNA stretch and chases the polymerase down to evict it.
Two bacterial stops: a self-folding hairpin, or a chasing Rho protein.
Both are bacterial mechanisms. Eukaryotes do not use Rho or simple hairpins for Pol II; their termination is coupled to RNA cleavage and polyadenylation, and is comparatively messy — the polymerase often runs well past the gene's true end before releasing.