Chemical modifications of core histones during exit of stationary phase in Saccharomyces cerevisae
The involvement of histone acetylation in facilitating gene expression is well-established, particularly in the case of histones H3 and H4. It was previously shown in Saccharomyces cerevisiae that gene expression was significantly down-regulated and chromatin more condensed in stationary phase compared to exponential phase. We were therefore interested in establishing the acetylation state of histone H3 and H4 in stationary and in exponential phase, since the regulation of this modification could contribute to transcriptional shut-down and chromatin compaction during semi-quiescence. We made use of nano-spray tandem mass spectrometry to perform a precursor ion scan to detect an m/z 126 immonium ion, diagnostic of an N -acetylated lysine residue that allowed unambiguous identification of acetylated as opposed to tri-methylated lysine. The fragmentation spectra of peptides thus identified were searched with Mascot against the Swiss-Prot database, and the y-ion and bion fragmentation series subsequently analyzed for mass shifts compatible with acetylated lysine residues. We found that K9, K14 and K36 of histone H3 and K12 and K16 of histone H4 were acetylated in exponential phase (bulk histones), but could not detect these modifications in histones isolated from stationary phase cells. The corresponding unacetylated peptides were, however, observed. This result was confirmed by Western analysis (work not presented here). H4K16 acetylation was previously shown to disrupt formation of condensed chromatin in vitro.