library_construction_protocol: RNA was extracted using the Qiagen miRNeasy kit, according to manufacturer's instructions For TT-seq samples, cells were treated with 4sU for 5 mins before RNA extraction as above For TT-seq samples, RNA was sonicated to ~1.5kb (Schwalb et al. 2016), then 4sU-labeled RNA was biotinylated (Duffy et al 2016) before isolation using streptavidin conjugated magnetic beads (Rädle et al. 2013) For mRNA-seq samples, libraries were prepared using the Illumina TruSeq Stranded mRNA Prep Kit, according to manufacturer's instructions For ribo-depleted RNA-seq samples, libraries were prepared using the Illumina Stranded Total RNA Prep Ligation with Ribo-Zero Plus, according to manufacturer's instructions