Polyadenylation Dependent Termination

Recognition of the polyadenylation signal provides the key positional cue for processing the nascent transcript. Once that signal has been identified, the RNA is cleaved downstream rather than at the signal itself. This placement separates the upstream transcript, which can receive a poly(A) tail, from the downstream RNA segment associated with termination.

Cleavage and polyadenylation are functionally linked because the upstream RNA fragment must acquire a poly(A) tail after cleavage to form an appropriate mature messenger RNA end. Coordinating these events connects transcript-end formation with the decision to stop transcription, supporting accurate gene expression rather than treating RNA maturation and termination as separate processes.

After cleavage, changes in RNA polymerase II conformation contribute to the transition from active transcription toward termination. These structural changes work alongside processing of the separated RNA fragments, helping polymerase stop after the transcript end has been defined. Their importance is that termination depends on both RNA processing and altered polymerase behavior.

XRN2 contributes by degrading the downstream RNA fragment in the 5′-to-3′ direction after cleavage. This exonucleolytic activity is part of the termination mechanism, rather than a consequence unrelated to transcript processing. Removing the downstream RNA helps promote polymerase release and limits continued transcription beyond the intended gene end.

The pathway proceeds through a linked sequence: RNA polymerase II transcribes a region containing the polyadenylation signal, the nascent RNA is cleaved downstream of that signal, and the upstream fragment receives a poly(A) tail. Polymerase conformational changes and 5′-to-3′ degradation of the downstream fragment then support transcriptional termination.

Accurate termination prevents transcriptional readthrough into neighboring genes, while the upstream RNA fragment can proceed toward mature messenger RNA production. This dual outcome connects transcript boundaries with genome organization and nuclear gene regulation. In biology, the pathway therefore helps preserve distinct gene expression units while coordinating RNA processing with transcription.