transcription termination

Knowing when to stop is as important as knowing when to start — otherwise the copyist keeps writing into the next recipe and the next, producing one giant useless scrawl. Transcription termination is the cell's full stop: the signal and the mechanism that make RNA polymerase release the finished RNA, let go of the DNA, and quit.

Bacteria use two main ways to stop. In intrinsic (or factor-independent) termination, the newly made RNA contains a sequence that folds back on itself into a stable hairpin loop followed by a run of uracils; the hairpin destabilizes the polymerase and the weak U-A pairs let the RNA slip free, and synthesis ends. In Rho-dependent termination, a protein called Rho latches onto the RNA and chases the polymerase down the transcript like a runner catching a train, finally prying the RNA loose when the enzyme pauses. Eukaryotes terminate differently and more loosely: termination is tied to RNA processing — for RNA polymerase II, a cleavage signal in the transcript triggers cutting and the addition of a poly-A tail, after which the polymerase eventually falls off downstream.

Termination defines the 3' boundary of a transcript, ensuring each gene yields a discrete RNA of the right length rather than a tangled read-through into neighbouring genes. It also offers another regulatory opportunity: in some bacteria, whether termination happens early (attenuation) decides how much of a gene gets transcribed at all. Getting the stop right is the difference between a clean usable message and molecular nonsense.