Recent highlights in understanding heterochromatin barrier activity
We have found that the heterochromatin barrier activity of the HS4 element is mediated by binding sites for the proteins USF1/USF2 and VEZF1 (CTCF binding is not required), which direct different mechanisms to counter the propagation of chromatin silencing. The USF transcription factors act to recruit several histone modifying enzymes that result in the acetylation of H3, H4 and H2A.Z and the methylation of H3K4 and H4R3. These so-called active histone modifications are proposed to collectively act as a chain terminator to heterochromatin assembly by interfering with the propagation of repressive histone modifications.