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Abstract
The chicken beta-globin 5'HS4 insulator element acts as a barrier to the encroachment
of chromosomal silencing. Endogenous 5'HS4 sequences are highly enriched with histone
acetylation and H3K4 methylation regardless of neighboring gene expression. We report
here that 5'HS4 elements recruit these histone modifications when protecting a reporter
transgene from chromosomal silencing. Deletion studies identified a single protein
binding site within 5'HS4, footprint IV, that is necessary for the recruitment of
histone modifications and for barrier activity. We have determined that USF proteins
bind to footprint IV. USF1 is present in complexes with histone modifying enzymes
in cell extracts, and these enzymes specifically interact with the endogenous 5'HS4
element. Knockdown of USF1 expression leads to a loss of histone modification recruitment
and subsequent encroachment of H3K9 methylation. We propose that barrier activity
requires the constitutive recruitment of H3K4 methylation and histone acetylation
at multiple residues to counteract the propagation of condensed chromatin structures.