PURIFICATION OF A NOVEL MHC CLASS-I ELEMENT-BINDING ACTIVITY FROM THYMUS NUCLEAR EXTRACTS REVEALS THAT THYMIC RBP-J-KAPPA CBF1 BINDS TO NF-KAPPA-B-LIKE ELEMENTS/

We purified a DNA binding protein that recognizes a portion of the MHC class I regulatory element region 1/NF-kappa B binding site whose exp ression correlates with the expression of a MHC class I transgene in t he thymus. The N-terminal amino acid sequence and the molecular size m atched the RBP-J kappa protein, also known as the EBV C-promoter bindi ng factor, CBF1. Antipeptide sera reactive with RBP-J kappa/CBF1 also reacted with this protein in gel mobility shift assays. Although RBP-J kappa/CBF1 is ubiquitously expressed, binding to the MHC class Ia NF- kappa B site was limited to the thymus. Comparison of the DNA binding specificities of RBP-J kappa/CBF1 in thymic and splenic nuclear extrac ts revealed strong binding from both extracts to an IFN-beta kappa B s ite containing the RBP-J kappa/CBF1 consensus sequence (CGTGGGAA). In contrast, only the thymic nuclear extract showed strong DNA binding ac tivity with probes containing the NF-kappa B recognition sequences pre sent in the MHC class Ia, IL-2R alpha, and granulocyte-macrophage CSF promoters. Thus, RBP-J kappa/CBF1 in thymic extracts demonstrates a cl early distinguishable DNA binding specificity that correlates with tis sue-specific expression of a class I transgene. This, coupled with the fact that our previous study showed enhanced expression of the transg ene in CD4(+)CD8(+) thymocytes, suggests that RBP-J kappa/CBF1 may pla y a role in the development of the immune system.