Inhibition experiments were performed in the presence of 0.6 mM MgCl2 and 50 mM Tris–HCl pH 7.5, conditions under which the reaction is fast and reaches a plateau of 65% cleavage in 10 min. 50 μl of RNA for a final concentration of 0.5 μM, was denaturated in buffer (50 mM Tris–HCl) at 90 °C for 1 min, slowly cooled (3 °C·min−1) to 23 °C, and then diluted in 400 μl of inhibitor solution containing the buffer.