Caspases 3 and 7: Key Mediators of Mitochondrial Events of Apoptosis

The structural effect of H4-K16Ac may di-rectly contribute to regions of decondensed chro-matin in eukaryotic orga nisms. In budding yeast,over 80% of H4 is acetyla ted at lysine 16, andmost of the genome exists in a decondensed state(3, 21). Likewise, evidence suggests that the tran-scriptionally enhanced X chromosome of maleflies, a site of ubiquitous H4-K16Ac, is decon-densed (22). Such decondensation of chromatinmay contribute to the establishment of transcrip-tionally active euchromatic regions. In vitro tran-scription studies suggest that the adoption ofhigher order chromatin structure reduces genetranscription (23). In contrast, acetylation ofH4-K16 increases gene transcription both in vitroandinvivo(24), and the decompaction resultingfrom such modification may increase the acces-sibility of factors that promote transcription.Do other histone marks regulate chroma-tin folding? The phosphorylation of H3-S10(S, serine) does not disrupt chromatin folding(25), and triacetylation of the H3 tail by Gcn5pdoes not disrupt chromatin compaction (26).Similarly, residues 1 to 13 of histone H4 thatinclude three sites of acetylation are dispens-able for folding of 601-177-12 arrays (9). Thus,H4-K16 is likely to be a unique acetylation siteof histone tails, which function as a dual switchfor higher order chromatin structure andprotein-histone interactions, promoting chroma-tin function in a mutually reinforcing manner.