We have been studying for some years the compound insulator at the 5 end of the chicken beta globin locus. We showed that in addition to its ability to prevent enhancer-promoter interactions, mediated by CTCF, it is also able to block the hetrochromatinization of a reporter gene. This property is independent of CTCF, but we have shown that it does depend on the binding of two other proteins, USF1 and BGP1. We have studied the role of USF1 extensively and shown that as part of its insulator function it recruits a wide variety of histone modifying enzymes, which serve to maintain nearby histones in an active state, and prevent other, repressive, histone modifications from being introduced. Our results show that USF1 interacts in vivo with the vertebrate Set1 complex, which methylates histone H3 at lysine 4, as well as PRMT1, which methylates histone H4 at arginine 3. These are present in two separate, multicomponent complexes which appear to be localized to the insulator element through specific binding of CTCF. These complexes are now being individually investigated, and the role particularly of PRMT1 is being studied at other sites in the genome where it is recruited by USF1.? We have also investigated the properties of BGP1, which binds to separate sites in the insulator. We find that the mouse homolog of BGP1 in mouse ES cells plays an important role in DNA methylation; in its absence methylation levels at critical sites genome-wide are depressed. We have made use of an ES cell line in which Vezf1, the mouse BGP1, is deleted. In collaboration with Dr. H. Stuhlmann (Cornell Medical College) we have now shown that in the absence of Vezf1 the DNA de novo methyl transferase, Dnmt3b, is down regulated. Wild type phenotype can largely be restored by introducing a Vezf1 expression vector into these cells. We have shown that Vezf1 binds on vivo to a site in an intron of the Dnmt3b gene.
| Huang, Suming; Li, Xingguo; Yusufzai, Timur M et al. (2007) USF1 recruits histone modification complexes and is critical for maintenance of a chromatin barrier. Mol Cell Biol 27:7991-8002 |
| Gaszner, Miklos; Felsenfeld, Gary (2006) Insulators: exploiting transcriptional and epigenetic mechanisms. Nat Rev Genet 7:703-13 |
