Practice question

Termination of RNA polymerase II transcription is coupled to three prime end processing and occurs after recognition of polyadenylation signal AAUAAA. CPSF-CstF machinery cleaves nascent RNA downstream of signal while polymerase continues transcribing one to two kilobases beyond cleavage site. Uncapped five prime monophosphate of downstream fragment becomes substrate for nuclear 5' to 3' exonuclease Xrn2 Rat1 torpedo that degrades RNA faster than elongation, eventually catching polymerase and triggering dissociation. Allosteric model also proposes conformational change after polyadenylation signal causing loss of elongation factors.