Flexible Stereospecific Interactions and Composition within Nucleoprotein Complexes Assembled on the TCRα Gene Enhancer

During thymocyte maturation, enhancers of genes encoding for TCRdelta (Tcrd) and TCRalpha (Tcra), Edelta(8), and Ealpha, work as a developmental switch controlling transition from Tcrd to Tcra activity at the Tcrad locus. Previous experiments revealed that an Ealpha fragment, Talpha1-Talpha2, which constitutes a well-characterized compact nucleoprotein structure led to premature activation of V(D)J recombination compared with that observed for the entire Ealpha or Talpha1-Talpha4. These experiments indicated that Talpha3-Talpha4 collaborates with factors bound to Talpha1-Talpha2 for the strict developmental regulation of Tcra rearrangement. The compact enhanceosome created on Talpha1-Talpha2 explained the molecular basis for requirement of intact Talpha2 TCF/LEF and ets sites for enhancer function. We have created a mutant version of Ealpha, EalphaMC, in which Edelta myb and runx sites have been substituted for Talpha2 runx and ets sites, that argues against the notion of a requirement for strict Ealpha enhanceosome structure for function. EalphaMC resulted in a very potent enhancer indicating that stereospecific interactions among proteins that form an Ealpha enhanceosome are rather flexible. Activation of V(D)J recombination by EalphaMC during thymocyte development resulted, however, to be premature and indistinguishable from that of Talpha1-Talpha2. These results indicate that Talpha3-Talpha4 itself is not sufficient to impart a developmental delay to a chimeric "early" enhancer, and indicate the need for functional collaboration between Talpha2 runx/ets sites binding proteins and proteins bound to Talpha3-Talpha4 for proper developmental activation. The possibility of assembly of distinct sets of proteins on Ealpha might represent a more flexible form of information processing during thymocyte development.