Epigenetics and transcription regulation during eukaryotic diversification: the saga of TFIID.

The basal transcription factor TFIID is central for RNA polymerase II-dependent transcription. Human TFIID is endowed with chromatin reader and DNA-binding domains and protein interaction surfaces. Fourteen TFIID TATA-binding protein (TBP)-associated factor (TAF) subunits assemble into the holocomplex, which shares subunits with the Spt-Ada-Gcn5-acetyltransferase (SAGA) coactivator. Here, we discuss the structural and functional evolution of TFIID and its divergence from SAGA. Our orthologous tree and domain analyses reveal dynamic gains and losses of epigenetic readers, plant-specific functions of TAF1 and TAF4, the HEAT2-like repeat in TAF2, and, importantly, the pre-LECA origin of TFIID and SAGA. TFIID evolution exemplifies the dynamic plasticity in transcription complexes in the eukaryotic lineage.

Xist-mediated chromatin changes that establish silencing of an entire X chromosome in mammals.

X chromosome inactivation (XCI) ensures an equal gene dosage between the sexes in placental mammals. Xist, a modular multi-domain X-encoded long non-coding RNA coats the X chromosome in cis during XCI. Xist recruits chromatin remodelers and repressor complexes ensuring silencing of the inactive X (Xi). Here, we review the recent work focused on the role of Xist functional repeats and interacting RNA-binding factors in the establishment of the silent state. Xist orchestrates recruitment of remodelers and repressors that first facilitate removal of the active chromatin landscape and subsequently direct the transition into a repressive heterochromatic environment. Some of these factors affect silencing on a chromosome-wide scale, while others display gene-specific silencing defects. The temporal order of recruitment shows each silencing step is party dependent on one another. After the Xi is established, many of the factors are dispensable, and a different repertoire of proteins ensure the silenced Xi is maintained and propagated.