“…) in glycerol buffer [25 % (w/v) glycerol, 5 mM MgCl 2 , 0.5 mM EGTA, 0.5 mM PMSF, 20 mM Tris/HCl (pH 7.4)] and exposed to transcription buffer [25 % (w/v) glycerol, 100 mM KCl, 5 mM MgCl 2 , 0.5 mM EGTA, 0.5 mM each ATP, CTP and GTP, 0.2 mM BrUTP (Sigma), 1 mM PMSF, 5 U RNAsin ml 21 (Promega), 50 mM Tris/HCl (pH 7.4)] for 30 min at 37 u C as described previously (Koberna et al, 2000). After incubation, the coverslips were fixed in cold 1 : 1 methanol : acetone mix at 220 u C for 4 min, dried and stained with anti-BrUTP monoclonal antibody (clone BU-33; Sigma) and Hoechst-33342 DNA dye as described previously (Bardina et al, 2009 The control cells displayed a moderate granular staining in the nucleoplasm that corresponded to RNA pol II-and pol III-dependent synthesis, which could not be discriminated by this method.…”