Fig 2: PRC1 influences Pol II modifications and Spt5 occupancy at active genes.TFIIH in the preinitiation complex (upper left) facilitates DNA unwinding to form the transcription bubble and phosphorylates Ser5 residues (Ser5P) in the heptad repeats in the CTD of the Rpb1 subunit of Pol II. Binding of Spt5 and NELF (negative elongation factor) pausing factors causes promoter-proximal pausing (upper right). The cohesin complex recruits PRC1 (green box), and the Sce subunit of PRC1 facilitates Ser5 phosphorylation and Spt5 binding by undefined mechanisms. Ph suppresses Ser5P at the promoter. P-TEFb in the SEC (super elongation complex) recruited by activator proteins phosphorylates Ser2 residues in the Pol II heptad repeats and Spt5 to start transcription elongation (bottom). Ph suppresses Spt5 association with elongating Pol II. The effects of PRC1 on Pol II and Spt5 coincide with changes, largely increases, in transcription of active genes.