Practice question

Torpedo model explains Pol II termination via cooperation between cleavage and exonucleolytic degradation. After endonucleolytic cut at polyadenylation site, downstream nascent RNA attached to elongating polymerase bears exposed five prime monophosphate lacking cap. Nuclear exonuclease Xrn2 in mammals, Rat1 in yeast, associates with Serine two phosphorylated CTD via Rtt103 and Pcf11, binds uncapped end, degrades RNA processively in five prime to three prime direction. Kinetic chase overtakes polymerase, destabilizes RNA-DNA hybrid and induces allosteric release of polymerase from template, completing termination and allowing recycling.