The transcription reaction was carried out in 50 ml containing 20 pmol of PCR product, 500 mM rNTPs, 3.3 mM guanosine, 40 U of ribonuclease inhibitor RNase Out (Invitrogen) and 400 U of T7 RNA polymerase (Ambion). The reaction was carried out at 378C for 2 h and the transcript was purified from a denaturing 10% polyacrylamide gel, and 5 0 -end-labeled with T4 polynucleotide kinase and [g 32 P]ATP (4500 Ci/mmol; ICN). The labeled RNA was again purified by electrophoresis on a denaturing 10% polyacrylamide gel.