Structural insights into Paf1 complex assembly and histone binding

The highly conserved Paf1 complex (PAF1C) plays critical roles in RNA polymerase II transcription elongation and in the regulation of histone modifications. It has also been implicated in other diverse cellular activities, including posttranscriptional events, embryonic development and cell survival and maintenance of embryonic stem cell identity. Here, we report the structure of the human Paf1/Leo1 subcomplex within PAF1C. The overall structure reveals that the Paf1 and Leo1 subunits form a tightly associated heterodimer through antiparallel beta-sheet interactions. Detailed biochemical experiments indicate that Leo1 binds to PAF1C through Paf1 and that the Ctr9 subunit is the key scaffold protein in assembling PAF1C. Furthermore, we show that the Paf1/Leo1 heterodimer is necessary for its binding to histone H3, the histone octamer, and nucleosome in vitro. Our results shed light on the PAF1C assembly process and substrate recognition during various PAF1C-coordinated histone modifications.