Efficient readout of posttranslational codes on the 50-residue tail of histone H3 by high-resolution MS/MS.

Histone modifications are highly linked to DNA methylation and together they exert epigenetic control over many activities in the cell including gene transcription. Using a streamlined mass spectrometric approach to determine changes in modification states in the first 50 residues of histone H3, we found a decrease in the global methylation states of H3.1 at Lys 9, Lys 14, and Lys 27 after inhibition of DNA methyltransferases by 5-aza-2'-deoxycytidine. Collisional ion dissociation methods proved adequate to determine site-specific H3 posttranslational modifications (PTMs) because ample backbone bonds are cleaved between each modification site and PTMs were stable to MS/MS using threshold fragmentation in a linear ion trap (LTQ). Our assay allows for a quick profiling and site-specific interrogation of modification states on the first 50 residues of H3 and is directly applicable to H3.1, H3.2, or H3.3 using most OrbiTrap, FT ICR, or TOF mass spectrometers.