Abstract
Previously, we characterized a DNA-binding protein, HS2NF5, that bound tightly to a conserved region within hypersensitive site 2 (HS2) of the human beta-globin locus control region (LCR) (Lam, L. T. , and Bresnick, E. H. (1996) J. Biol. Chem. 271, 32421-32429). The beta-globin LCR controls the chromatin structure, transcription, and replication of the beta-globin genes. We have now purified HS2NF5 to near-homogeneity from fetal bovine thymus. Two polypeptides of 56 and 61 kDa copurified with the DNA binding activity. The two proteins bound to the LCR recognition site with an affinity (3.1 nM) and specificity similar to mouse erythroleukemia cell HS2NF5. The amino acid sequences of tryptic peptides of purified HS2NF5 revealed it to be identical to the murine homolog of the suppressor of hairless transcription factor, also known as recombination signal binding protein Jkappa or C promoter binding factor 1 (CBF1). The CBF1 site within HS2 resides near sites for hematopoietic regulators such as GATA-1, NF-E2, and TAL1. An additional conserved, high affinity CBF1 site was localized within HS4 of the LCR. As CBF1 is a downstream target of the Notch signaling pathway, we propose that Notch may modulate LCR activity during hematopoiesis.
