RNase H cleavage was performed essentially as described (35) . Samples of 10 mg of total RNA were annealed with 40 ng of oligonucleotide complementary to the specific regions within rRNA at 688C for 10 min and digested with 1.5 U RNase H at 308C for 1 h. For detection, samples were separated on polyacrylamide gels and analysed by northern hybridization using probes located upstream of RHase H cleavage.