From the data presented so far, it appears that the observed rRNA fragmentation may possibly represent a part of the cellular oxidative stress and apoptotic responses. This was assessed by testing the extent of the 25S rRNA degradation in different mutants defective in these pathways. In the first place, yca1D strain, lacking the only identified apoptotic metacaspase Yca1 in yeast, and aif1D cells not expressing the yeast apoptosis inducing factor Aif1 (13,15), were assayed for H 2 O 2 -induced RNA decay, however, no significant differences were observed (Supplementary Figure S1B and C) . Similarly, addition of a broad-range caspase inhibitor z-VAD-FMK (20 mM) that prevents Yca1-dependent cell death in yeast (13, 48) had no effect on rRNA fragmentation (data not shown). This indicates that rRNA degradation detected in all apoptotic conditions tested is independent of the two major apoptosis mediators, Yca1 and Aif1, and of other potential yeast caspases. Likewise, treatment with translation inhibitor cycloheximide, that has been shown to prevent apoptotic cell death induced by H 2 O 2 and acetic acid (29, 36) , had little or no effect on H 2 O 2 -mediated rRNA degradation (Supplementary Figure S1A) . However, it appears that events in the course of apoptosis that require protein synthesis are rather late, for example DNA fragmentation and chromatin condensation, whereas rRNA decay is initiated relatively fast. In contrast, different outcome was observed for mutants inhibiting chromatin condensation during H 2 O 2 -induced apoptosis. Phosphorylation of Serine 10 and deacetylation of Lysine 11, both in histone H2B, were reported to have an essential role for the progress of cell death in yeast (20, 21) . In agreement, S10A or K11Q mutations in histone H2B that prevent these modifications and abrogate apoptosis resulted in the significant reduction of the 25S rRNA degradation, both the decay of the mature rRNA and the amount of degradation products ( Figure 5A ). Together, these data show that the destruction of ribosomal RNA in cells treated with H 2 O 2 , and possibly with other apoptotic stimuli, is a part of a yeast cell death pathway that involves histone modification and not the caspase-dependent pathway. Remarkably, Nuc1-mediated apoptosis resulting from over-expression of yeast EndoG homologue Nuc1, a major mitochondrial nuclease, was also reported to be Yca1-and Aif1-independent and related to histone modifications (18) . Alternatively, it can be envisaged that damage of ribosomes triggered by apoptotic stimuli is an early event during the response, does not require protein synthesis, precedes caspase activation and acts as an upstream signal in the apoptotic pathway. This scenario is consistent with most observations so far.