LEDGF/p75 promotes transcriptional pausing through preventing SPT5 phosphorylation.

SPT5 exhibits versatile functions in RNA Pol II promoter proximal pausing, pause release, and elongation in metazoans. However, the mechanism underlying the functional switch of SPT5 during early elongation has not been fully understood. Here, we report that the phosphorylation site-rich domain (PRD)/CTR1 and the prion-like domain (PLD)/CTR2, which are situated adjacent to each other within the C-terminal repeat (CTR) in SPT5, play pivotal roles in Pol II pausing and elongation, respectively. Our study demonstrates that LEDGF/p75 is highly enriched at promoters, especially paused promoters, and prevents the phosphorylation of SPT5 PRD by the super elongation complex (SEC). Furthermore, deletion of LEDGF IBD leads to increased SEC occupancies and SPT5 PRD phosphorylation at promoters and also increased pause release. In sum, our study reveals that LEDGF and SEC function cooperatively on SPT5 distinct domains to ensure proper transcriptional transition from pausing to elongation.