The role of Mediator and Little Elongation Complex in transcription termination

Mediator is a coregulatory complex involved in regulating the transcription of Pol II-dependent genes. Metazoan Mediator subunit MED26 functions as a docking site for the ELL/EAF-containing Super Elongation Complex (SEC) and L ittle Elongation Complex (LEC), which regulate the expression of distinct genes. MED26 helps to recruit SEC to protein-coding genes including c-Myc and LEC to small nuclear RNA (snRNA) genes. However, why MED26 takes advantage of SEC or LEC to regulate different claases of genes is unclear. Here, we present evidence that MED26 recruits LEC to support optimal transcription termination at non-polyadenylated genes including snRNA and replication-dependent histone (RDH) genes. Our findings indicate that LEC recruited by MED26 promotes efficient transcription termination by Pol II through interaction with CBC-ARS2 and NELF/DSIF, and then LEC promotes 3'-end processing through the recruitment of Integrator or Heat labile factor to snRNA or RDH genes, respectively. Overall design: Experiment 1: Rpb1 ChIP-seq HEK293T WT and MED26 mutant cells, using mouse chromatin as spike-in. Experiment 2: MED26 ChIP-seq in HEK293T WT cells. Experiment 3: PRO-seq in HEK293T WT and MED26 mutant cells, using D. melanogastor Kc167 cells as spike-in control. Experiment 4: RNA-seq of polyA-selected libraries prepared from HEK293T cells treated with control, AFF4, KIAA0947/ICE1, or MED26 siRNAs. Experiment 5: RNA-seq of ribo-depleted libraries prepared from HEK293T cells treated with control or MED26 siRNAs. Experiment 6: RNA-seq of ribo-depleted libraries prepared from HEK293T WT or MED26 mutant cells.