library_construction_protocol: RNA was extracted with RNeasy miniprep kit (Qiagen). For SLAM-seq, 5 µg RNA of 4sU laballed RNA for each sample was subjected to alkylation by 100 mM iodoacetamide. 1 µg of RNA of each sample was used for libarary synthesis with QuantSeq 3' mRNA-Seq Library Prep Kit FWD for Illumina (Lexogen, #015) following manufacturer's protocols. 3' end mRNA-seq.