GSM7156317
cells were lysed in 1ml cold Cell Lysis Buffer (10mM Tris pH 8, 10mM NaCl, 0.2% NP-40/Igepal) for 10min on ice. Nuclei were pelleted at 4℃ and washed once with cold 1.4 × NEB buffer 3.1 (NEB Cat#B7203S). Nuclei were then re-suspended in 25ul of 1.4 × NEB buffer 3.1 and treated with 0.1% SDS for 10min at 65℃. Nuclei were then treated with 1% Triton X-100 to quench SDS for 1h at 37℃. Chromatin was digested with 25U DpnII restriction enzyme (NEB, Cat#R0543M) at 37℃ overnight with shaking (950rpm on a thermal mixer). After overnight digestion, an additional stroke of 25U DpnII was added into the system to further digest the genome for 4 hours at 37℃. After the 2nd stroke of digestion, nuclei were incubated at 65℃ for 20min to inactivate DpnII. Digested chromatin fragments were then cooled down to room temperature and blunted with dCTP, dGTP, dTTP (Diamond Cat#B110049, B110050, B110051) and Biotin-14-dATP (active motif, Catalog No: 14138) using 6.6U DNA Polymerase I, Large (Klenow) fragment (NEB, Cat#M0210). Chromatin was then ligated in-situ with 2000U T4 DNA ligase (NEB, Cat#M0202M) for 4 hours at 16℃ followed by further incubation for 2 hours at room temperature. Chromatin was then reverse-crosslinked overnight at 65℃ in the presence of 1% SDS and proteinase K (G-clone Cat#EZ0970). After reverse-crosslinking, DNase-free RNase A (Vazyme Cat#DE111-01) was added to the system for 30min at 37℃ to remove RNA. DNA was then extracted by phenol-chloroform extraction, precipitated, and dissolved in 130ul of nuclease free water. DNA was sonicated to 200-300bp fragments with a QSonica Q800R3 sonicator (40% amplitude, 15s ON and 15s OFF, 13min sonication) and purified with VAHTS DNA Clean Beads (Vazyme Cat#N411). Biotin-labeled DNA fragments was enriched by incubation with 100μl Dynabeads MyOne Streptavidin C1 beads (Thermal Fisher Scientific, Cat#65002) at room temperature for 15min. Streptavidin C1 beads were then washed twice with 1 × B&W buffer (5mM Tris-HCl, pH 7.5; 0.5mM EDTA; 1M NaCl). Beads were re-suspended in 30ul of 1 × NEB Buffer2 (NEB Cat#7002S) containing 9U of T4 DNA polymerase (NEB, Cat#M0203S) and 40uM dATG/dGTP and incubated at 20℃ for 4 hours to remove unligated fragments. Beads were then washed twice with 1 × B&W buffer and subject to library construction. DNA end repair, dA-tailing and adaptor ligation were constructed on beads, using the AHTS Universal DNA Library Prep Kit for Illumina or MGI (Vazyme, Cat# ND627-01, NDM607-02) based on the manufacture's protocol. To eluted DNA from streptavidin beads, the library mixture was treated with 0.1% SDS and incubated at 98℃ for 10min. Released DNA was collected from the supernatant and purified with VAHTS DNA Clean Beads. Finally, the library was amplified for 9 cycles on a thermal cycler, using the VAHTS® HiFi Amplification Mix. PCR products were then purified with VAHTS DNA Clean Beads and sequenced on an Illumina NextSeq500 or MGI DNBSEQ-T7 sequencing platform.