Activation of a T-box-Otx2-Gsc gene network independent of TBP and TBP-related factors

Abstract

The developmental program relies on activating and repressing regulatory influences which are faithfully integrated at the core promoter of individual genes. In vertebrates, the basal machinery recognizing the core promoter includes TATA binding protein (TBP) and two TBP-related factors. In Xenopus embryos the three TBP family factors are all essential for development and required for expression of distinct subsets of genes. Here we report on a non-canonical TBP family-insensitive (TFI) mechanism of transcription initiation that involves mesoderm and organizer gene expression. Using TBP family single and triple knockdown experiments, α-amanitin treatment, transcriptome profiling and chromatin immunoprecipitation, we found that TFI gene expression cannot be explained by functional redundancy, is supported by active transcription, and shows normal recruitment of the initiating form of RNA polymerase II to the promoter. Strikingly, recruitment of Gcn5 (Kat2a), a co-activator which has been implicated in transcription initiation, to TFI promoters is increased upon depletion of TBP family factors. TFI genes are part of a densely connected TBP family-insensitive T-box-Otx2-Gsc interaction network. The results indicate that this network of genes bound by Vegt, Eomes, Otx2 and Gsc utilizes a novel, flexible and non-canonical mechanism of transcription that does not require TBP or TBP-related factors.