Prior to structure probing reactions, the labeled RNA was subjected to a denaturation and renaturation procedure in a buffer containing 2 mM MgCl 2 , 80 mM NaCl, 20 mM Tris-HCl pH 7.2 by heating the sample at 808C for 1 min. and then slowly cooling to reaction temperature. Limited RNA digestion was initiated by mixing 5 ml of the RNA sample (50 000 c.p.m.) with 5 ml of a probe solution containing lead ions, nuclease S1 or ribonucleases T1, T2 or Cl3. The reactions were performed at 378C for 10 min. and stopped by adding an equal volume of stop solution (7.5 M urea and 20 mM EDTA with dyes) and sample freezing.