Abstract
Transcription initiation is a major regulatory step in eukaryotic gene expression. Co-activators establish transcriptionally competent promoter architectures and chromatin signatures to allow the formation of the pre-initiation complex (PIC), comprising RNA polymerase II (Pol II) and general transcription factors (GTFs). Many GTFs and co-activators are multisubunit complexes, in which individual components are organized into functional modules carrying specific activities. Recent advances in affinity purification and mass spectrometry analyses have revealed that these complexes often share functional modules, rather than containing unique components. This observation appears remarkably prevalent for chromatin-modifying and remodeling complexes. Here, we use the modular organization of the evolutionary conserved Spt-Ada-Gcn5 acetyltransferase (SAGA) complex as a paradigm to illustrate how co-activators share and combine a relatively limited set of functional tools. Modular Organization of Transcription Co-Activator Complexes Formation of the transcription PIC, containing Pol II and GTFs, is a major regulatory step in eukaryotic gene expression. PIC formation and accessibility of template DNA to the transcription machinery is controlled by the degree of chromatin compaction. Co-activator complexes have critical roles in this process, by regulating PIC formation through direct interactions with basal transcription factors, by ATP-dependent nucleosome remodeling, and by covalent modification of histones. Chromatin-modifying co-activators dynamically deposit or remove post-translational modifications (PTMs) on histones, creating or erasing docking surfaces for specific regulatory factors ([1,2] and references therein).