show Abstracthide AbstractSAGA is a modular cofactor complex that is essential for eukaryotic transcription. SAGA's complement of ~20 proteins exist within four structurally and functionally distinct modules, two of which are catalytic. Within the KAT module, GCN5 acetylates histone tails, leading to increased chromatin accessibility and bromodomain protein recruitment. The DUB module contains the ubiquitin hydrolase USP22. In yeast, the USP22 ortholog deubiquitylates H2B, resulting in Pol II S2 phosphorylation and subsequent transcriptional elongation. We report here that metazoan SAGA, and USP22 specifically, are required at a more proximal stage in activator-driven transcription, i.e. pre-initiation complex (PIC) assembly. A combination of genome-wide and proteomic analyses revealed that H2B deubiquitylation is not linked to USP22-dependent transcription. Instead, USP22 controls Mediator tail subunit ubiquitylation. Mechanistically, USP22 controls loading of Mediator tail and GTFs onto promoters, with Mediator core recruitment being USP22-independent. These findings place human SAGA function at the earliest steps in activator-driven transcription. Overall design: CHIP-seq for various factors in USP22 wild type and knockdown conditions before and after ER stress response