DNA-directed termination of mammalian RNA polymerase II.

The best-studied mechanism of eukaryotic RNA polymerase II (RNAPII) transcriptional termination involves polyadenylation site-directed cleavage of the nascent RNA. The RNAPII-associated cleavage product is then degraded by XRN2, dislodging RNAPII from the DNA template. In contrast, prokaryotic RNAP and eukaryotic RNAPIII often terminate directly at T-tracts in the coding DNA strand. Here, we demonstrate a similar and omnipresent capability for mammalian RNAPII. Importantly, this termination mechanism does not require upstream RNA cleavage. Accordingly, T-tract-dependent termination can take place when XRN2 cannot be engaged. We show that T-tracts can terminate snRNA transcription independently of RNA cleavage by the Integrator complex. Importantly, we found genome-wide termination at T-tracts in promoter-proximal regions but not within protein-coding gene bodies. XRN2-dependent termination dominates downstream from protein-coding genes, but the T-tract process is sometimes used. Overall, we demonstrate global DNA-directed attrition of RNAPII transcription, suggesting that RNAPs retain the potential to terminate over T-rich sequences throughout evolution.