USF1 Recruits hSET1 Complex and Is Important for Maintaining Active Chromatin Domains in the β-Globin Locus.

The chicken β-globin insulator element acts as a barrier to the encroachment of chromosomal silencing. This barrier insulator prevents the spread of a 16 Kb domain of condensed chromatin which lies immediately upstream of the β-globin genes in the chicken genome. We have previously shown that the transcription factors USF1/2 mediate the barrier activity by binding to the 5′HS4 insulator element of the chicken β-globin locus and maintaining a local active chromatin structure. In the mouse β-globin locus, USFs also play a critical role in regulating the developmental stage-specific transcription of the β-major globin gene. To further understand the diverse roles of USF proteins, we undertook the biochemical isolation of USF1-containing multiprotein complexes. We found that USF1 associates with the hSET1 complex, which is an H3K4-specific methyltransferase. This complex exhibits histone H3 methyltransferase enzymatic activity. Consistent with the role of USF1 in recruiting histone modifying enzyme hSET1 to the 5′HS4 insulator, knockdown of USF1 expression in erythroid leukemia cell line 6C2 leads to the loss of histone H3K4 dimethylation at 5′HS4 of the chicken β-globin locus. In addition, USF1, hSET1, as well as H3K4 methylation are colocalized on HS2 of the locus control region (LCR) and the promoter of the β-major globin gene. Furthermore, the double ChIP analysis demonstrates that the histone methyltransferase hSET1 is recruited by transcription factor USF1 onto the mouse β-globin locus. The results suggest that hSET1 may play an important role in establishing H3K4 methylations on the active β-globin locus and reveal the mechanisms of USF and its associated proteins in maintaining the open chromatin domain and in regulation of globin gene transcription.