An unexpected role of TAFs and TRFs in skeletal muscle differentiation: switching core promoter complexes

Sequence-specific enhancer-binding transcription factors and chromatin-modifying proteins are well recognized for their potential contributions to cell-type-specific gene regulation. In contrast, the role of core promoter recognition factors, such as TFIID in modulating gene- and cell-type-specific programs of transcription has been less understood. In general, the so-called basal factors have largely been relegated to a supporting role as invariant components of the preinitiation complex. To dissect the potential contributions of TFIID to cell-type-specific transcription, we have studied the developmental process of skeletal myogenesis. Terminal differentiation during myogenesis involves an intricate reprogramming of transcription that is thought to be directed by cell-type-specific transcription regulatory factors. Here, we summarize our findings that the canonical TFIID complex must first be dismantled as a requisite step during the differentiation of myoblasts into myotubes and subsequently substituted by a novel core transcription complex composed of TAF3 and TRF3. Although this remarkable mechanism of completely switching core promoter recognition complexes to drive terminal differentiation has not been previously documented, it may eventually prove to be the rule rather than the exception as we learn more about cell-type-specific gene regulation.

Switching of the core transcription machinery during myogenesis

Transcriptional mechanisms that govern cellular differentiation typically include sequence-specific DNA-binding proteins and chromatin-modifying activities. These regulatory factors are assumed necessary and sufficient to drive both divergent programs of proliferation and terminal differentiation. By contrast, potential contributions of the basal transcriptional apparatus to orchestrate cell-specific gene expression have been poorly explored. In order to probe alternative mechanisms that control differentiation, we have assessed the fate of the core promoter recognition complex, TFIID, during skeletal myogenesis. Here we report that differentiation of myoblast to myotubes involves the disruption of the canonical holo-TFIID and replacement by a novel TRF3/TAF3 (TBP-related factor 3/TATA-binding protein-associated factor 3) complex. This required switching of core promoter complexes provides organisms a simple yet effective means to selectively turn on one transcriptional program while silencing many others. Although this drastic but parsimonious transcriptional switch had previously escaped our attention, it may represent a more general mechanism for regulating cell type-specific terminal differentiation.