USF1 Recruits Histone Modification Complexes and Is Critical for Maintenance of a Chromatin Barrier

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Abstract
The insulator element at the 5′ end of the chicken β-globin locus acts as a barrier, protecting transgenes against silencing effects of adjacent heterochromatin. We showed earlier that the transcription factor USF1 binds within the insulator and that this site is important for generating in adjacent nucleosomes histone modifications associated with active chromatin and, by inference, with barrier function. To understand the mechanism of USF1 action, we have characterized USF1-containing complexes. USF1 interacts directly with the histone H4R3-specific methyltransferase PRMT1. USF1, PRMT1, and the histone acetyltransferases (HATs) PCAF and SRC-1 form a complex with both H4R3 histone methyltransferase and HAT activities. Small interfering RNA downregulation of USF1 results in localized loss of H4R3 methylation, and other histone modifications associated with euchromatin, at the insulator. A dominant negative peptide that interferes with USF1 binding to DNA causes silencing of an insulated reporter construct, indicating abolition of barrier function. These results show that USF1 plays a direct role in maintaining the barrier, supporting a model in which the insulator works as a barrier by maintaining a local environment of active chromatin.