COOPERATION AMONG MULTIPLE TRANSCRIPTION FACTORS IS REQUIRED FOR ACCESS TO MINIMAL T-CELL RECEPTOR ALPHA-ENHANCER CHROMATIN IN-VIVO

To understand the molecular basis for the dramatic functional synergy between transcription factors that bind to the minimal T-cell receptor alpha enhancer (E alpha), we analyzed enhancer occupancy in thymocyte s of transgenic mice in vivo by genomic footprinting. We found that th e formation of a multiprotein complex on this enhancer in vivo results from the occupancy of previously identified sites for CREB/ATF, TCF/L EF, CBF/PEBP2, and Ets factors as well as from the occupancy of two ne w sites 5' of the CRE site, GC-I (which binds Sp1 in vitro) and GC-II. Significantly, although all sites are occupied on a wild-type E alpha , all sites are unoccupied on versions of E alpha with mutations in th e TCF/LEF or Ets sites. Previous in vitro experiments demonstrated hie rarchical enhancer occupancy with independent binding of LEF-1 and CRE B, Our data indicate that the formation of a multiprotein complex on t he enhancer in vivo is highly cooperative and that no single E alpha b inding factor can access chromatin in vivo to play a unique initiating role in its assembly. Rather, the simultaneous availability of multip le enhancer binding proteins is required for chromatin disruption and stable bidding site occupancy as well as the activation of transcripti on and V(D)J recombination.