BackgroundInformation | HistonesarehighlyconservedproteinsthatserveasthestructuralscaffoldfortheorganizationofnuclearDNAintochromatin.HistonesmodificationsregulateDNAtranscription,repair,recombination,andreplication.Themostcommonlystudiedmodificationsareacetylation,phosphorylation,methylation,andubiquitination.Thesemodificationscanalterlocalchromatinarchitecture,orrecruittrans-actingfactorsthatrecognizespecifichistonemodifications(the"histonecode"hypothesis).MethylationofhistoneH4atlysine20hasbeenimplicatedintranscriptionalactivation,genesilencing,heterochromatinformation,mitosis,andDNArepair.HistoneH4isprogressivelymethylatedatLys20duringtheG(2),M,andG(1)phasesofthecellcycle,andmethylationofH4Lys20isaMarkerforheterochromatin.Inmammals,thefemaleXchromosomeiscoatedbytheXistRNA,whichiscriticalforsilencing,andismarkedmainlybymethylationofH3Lys9,H3Lys27andH4Lys20. |