Abstract
To directly and quantitatively identify the transcriptional protein complexes assembled on accessible chromatin, we develop an assay for transposase-accessible chromatin using mass spectrum (ATAC-MS) based on direct transposition of biotinylated adaptors into open chromatin. Coupling with activated gene sequence information by ATAC-seq, ATAC-MS can profile the accessible chromatin-protein machinery. ATAC-MS, combined with fractionation strategies (fATAC-MS), can provide a high-resolution chromatin-transcriptional machinery atlas. ATAC-MS with a novel Tn5-dCas9 fusion protein [dCas9-targeted ATAC-MS (ctATAC-MS)] further facilitates systematic pinpointing of the transcriptional machinery at specific open chromatin regions. We used ATAC-MS and ATAC-seq to investigate transcriptional regulation during C2C12 cell differentiation and demonstrated the role of RFX1 in regulating the proliferation and differentiation of C2C12 cells. Our strategy provides a universal toolbox including ATAC-MS, fATAC-MS, and ctATAC-MS, which enables us to portray the transcriptional regulation machinery atlas in genome scale and investigate the protein-DNA complex at a specific genomic locus.