Breaking through to the other side: silencers and barriers

The establishment and restriction of transcriptionally inactive regions in the nucleus is mediated by silencer and barrier elements. Silencer-bound proteins recruit additional factors to establish the silenced domain during the S-phase of the cell cycle but, contrary to previous models, DNA replication is not a pre-requisite for the establishment. Characteristically, silenced domains contain hypoacetylated histones and recent data have identified residue-specific methylation of histone H3 as an additional signature that distinguishes active regions from inactive ones. Peaks of acetylated histones demarcate the boundaries between these regions and recruitment of HAT activities provides a mechanism to restrict the spread of heterochromatin.