Human 5′→3′ exonuclease Xrn2 promotes transcription termination at co-transcriptional cleavage sites

Eukaryotic protein-encoding genes possess poly(A) signals that define the end of the messenger RNA and mediate downstream transcriptional termination by RNA polymerase II (Pol II)(1). Termination could occur through an 'anti-termination' mechanism whereby elongation factors dissociate when the poly(A) signal is encountered, producing termination-competent Pol II2,3. An alternative 'torpedo' model postulated that poly(A) site cleavage provides an unprotected RNA 5' end that is degraded by 5' --> 3' exonuclease activities ( torpedoes) and so