The purified 59-triphosphorylated RNA was further processed to obtain a diphosphorylated 59 end using the S. cerevisiae RNA 59triphosphatase (Cet1) which was expressed and purified as described before [33] . The diphosphorylated RNA (ppRNA) was precipitated with ethanol, resuspended, quantitated by spectrophotometry, and stored at 220uC.