PAX5 (BSAP) REGULATES THE MURINE IMMUNOGLOBULIN 3'ALPHA ENHANCER BY SUPPRESSING BINDING OF NF-ALPHA-P, A PROTEIN THAT CONTROLS HEAVY-CHAIN TRANSCRIPTION

The Pad transcription factor BSAP (B-cell-specific activator protein) is known to bind to and repress the activity of the immunoglobulin hea vy chain 3'alpha enhancer, We have detected an element-designated alph a P-that lies approximate to 50 bp downstream of the BSAP binding site 1 and is required for maximal enhancer activity, In vitro binding exp eriments suggest that the 40-kDa protein that binds to this element (N F-alpha P) is a member of the Ets family present in both B-cell and pl asma-cell nuclei. However, in Five footprint analysis suggests that th e alpha P site is occupied only in plasma cells, whereas the BSAP site is occupied in B cells but not in plasma cells. When Pax5 binding to the enhancer in B cells Has blocked in vivo by transfection with a tri ple-helix-forming oligonucleotide, an alpha P footprint appeared and e ndogenous immunoglobulin heavy chain transcripts increased, The tripie -helix-forming oligonucleotide also increased enhancer activity of a t ransfected construct in B cells, but only when the alpha P Site was in tact. Pax5 thus regulates the 3'alpha enhancer and immunoglobulin gene transcription by blocking activation by NF-alpha P.