title: GSM3184723: GRO-seq, siCTL_E2 (repeat1) MCF7; Homo sapiens; OTHER
library_construction_protocol: Methods: The transfected MCF7 cells were subjected to GRO-seq experiments. GRO-Seq experiments were performed as previously reported (Li et al., 2013, Nature). Briefly, ~10-20 millions of MCF-7 cells were washed 3 times with cold PBS and then sequentially swelled in swelling buffer (10mM Tris-Cl pH7.5, 2mM MgCl2, 3mM CaCl2) for 5 min on ice, harvested, and lysed in lysis buffer (swelling buffer plus 0.5% NP-40 and 10% glycerol). The resultant nuclei were washed one more time with 10mL lysis buffer and finally re-suspended in 100uL of freezing buffer (50mM Tris-Cl pH8.3, 40% glycerol, 5mM MgCl2, 0.1mM EDTA). For the run-on assay, re-suspended nuclei were mixed with an equal volume of reaction buffer (10mM Tris-Cl pH 8.0, 5mM MgCl2, 1mM DTT, 300mM KCl, 20 units of SUPERase-IN, 1% sarkosyl, 500uM ATP, GTP, and Br-UTP, 2uM CTP) and incubated for 5 min at 30°C . The resultant nuclear-run-on RNA (NRO-RNA) was then extracted with TRIzol LS reagent (Life Technologies) following manufacturer's instructions. NRO-RNA was fragmented to ~300-500nt by alkaline base hydrolysis on ice and followed by treatment with DNase I and Antarctic phosphatase. These fragmented Br-UTP labeled nascent RNA was then immunoprecipitated with an anti-BrdU argarose beads (Sc32323ac, Santa Cruz Biotechnology) in binding buffer (0.5XSSPE, 1mM EDTA, 0.05% tween) for 3 hrs at 4°C with rotation. Subsequently, T4 PNK was used to repair the end of the immunoprecipitated BrU-NRO-RNA, at 37°C for 1hr. The RNA was extracted and precipitated using acidic phenol-chloroform.    cDNA synthesis was performed as per a published method (54) with few modifications. The RNA fragments were subjected to poly-A tailing reaction by poly-A polymerase (NEB) for 30 min at 37°C. Subsequently, reverse transcription was performed using oNTI223 primer and superscript III RT kit (Life Technologies). The cDNA products were separated on a 10% polyacrylamide TBE-urea gel and only those migrated between ~100-500bp were excised and recovered by gel extraction. After that, the first-strand cDNA was circularized by CircLigase (Epicentre) and re-linearized by APE1 (NEB). Re-linearized single strand cDNA (sscDNA) was separated by a 10% polyacrylamide TBE gel as described above and the product of needed size was excised (~170-400bp) for gel extraction. Finally, sscDNA template was amplified by PCR (usually between 10-14 PCR cycles) using the Phusion High-Fidelity enzyme (NEB) according to the manufacturer's instructions. The resulted library was subjected to deep sequencing.