TCR alpha enhancer activation occurs via a conformational change of a pre-assembled nucleoprotein complex

The TCR alpha enhancer (E alpha) has served as a paradigm for studying how enhancers organize trans-activators into nucleo-protein complexes thought t o recruit and synergistically stimulate the transcriptional machinery. Litt le is known, however, of either the extent or dynamics of E alpha occupancy by nuclear factors during T cell development. Using dimethyl sulfate (DMS) irt five footprinting, we demonstrate extensive Ea occupancy, encompassing both previously identified and novel sites, not only in T cells representi ng a developmental stage where E alpha is known to be active (CD4(+)CD8(+)- DP cells), but surprisingly, also in cells at an earlier developmental stag e where E alpha is not active (CD4(-)CD8(-)-DN cells). partial occupancy wa s also established in B-lymphoid but not non-lymphoid cells. Irt vivo DNase I footprinting, however, implied developmentally induced changes in nucleo -protein complex topography. Stage-specific differences in factor compositi on at E alpha sequences were also suggested by EMSA analysis. These results , which indicate that alterations in the structure of a pre-assembled nucle o-protein complex correlate with the onset of E alpha activity, may exempli fy one mechanism by which enhancers can rapidly respond to incoming stimuli .