Cleavages in the 25S rRNA are endonucleolytic and require cellular machinery Specific cleavages within the 25S rRNA generated in the presence of hydrogen peroxide were monitored by northern hybridization with probes located along the molecule to narrow down the regions to be further analysed ( Figure 2A ). This analysis showed the accumulation of diverse degradation products, some of which extended from the 5 0 -end of the molecule (Figure 2A, lanes 1-6) , whereas others were also truncated at their 5 0 ends ( Figure 2A, lanes 25-30) . The striking decrease in the level of the mature 25S rRNA at higher doses of the oxidant (1-3 mM) indicates that following specific cleavages the majority of rRNA becomes degraded, possibly by the exosome complex of 3 0 !5 0 exonucleases that participates in the decay of rRNA precursors and excised transcribed spacers (41) . The emergence of the characteristic cut-off in the signal at the fragment size corresponding to the position of the probe indicated that major cleavage sites are located around positions +400, +600 and +900 with respect to the 5 0 -end of the molecule. Two of these cleavages, at positions +610-611 and +398-403, were mapped for treatment with 1 mM H 2 O 2 by primer extension using primers W237 and W235 situated downstream of the expected cleavage sites ( Figure 2B ). Similarly, major cleavage sites were analysed in 16-day old chronologically aged cells using the same primers ( Figure 2C ) and mapped at positions +601-602 and +478-501. According to the secondary structure of the 25S rRNA taken from (42), the regions where mapped cleavages occur (shown in Figure 2B and C besides corresponding primer extension reactions) are located at unpaired nucleotides in loops or bulges. This points to the action of single-stranded RNA nucleases.