HMG BOX-ACTIVATING FACTOR-1 AND FACTOR-2, 2 HMG BOX TRANSCRIPTION FACTORS THAT BIND THE HUMAN IG HEAVY-CHAIN ENHANCER

We present the isolation of two cDNAs that encode distinct, yet relate d, proteins that bind the HE2 region of the human Ig heavy chain (IgH) enhancer, Designated HMG box-activating factors (HAF) 1 and 2, the tw o proteins are new members of the HMG box family of DNA binding protei ns, Both are potent transcription activators when expressed 1) as GAL4 fusions targeted to promoters containing GAL4 operators, or 2) as int act proteins targeted to minimal promoters containing binding sites de rived from the IgH enhancer, HAF-1 and HAF-2 mRNAs are apparently expr essed in both B cells and non-B cells, However, activity generated by the isolated HE2 region in B cells is dependent on both an intact HAF- 1/HAF-2 binding site and at least one additional site that has been re ported previously to bind a B cell-restricted protein, Our results sug gest a collaborative role for either or both HAF-1 and HAF-2 in establ ishing the B cell activity of the human IgH enhancer.