The ubiquitin hydrolase USP22 contributes to 3'‐end processing of JAK‐STAT‐inducible genes

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The JAK-STAT (Janus kinase-signal transducer and activator of transcription) signaling pathway drives cellular growth, differentiation, and the immune response. STAT-activated gene expression is both rapid and transient and requires dynamic post-translational modification of the chromatin template. We previously showed that monoubiquitination of histone H2B (ubH2B) is highly dynamic at the STAT1 target gene, interferon regulatory factor 1 (IRF1), suggesting that a deubiquitinase is recruited during gene activation. Here, we report that RNAi-mediated knockdown of the ubiquitin hydrolase, USP22, results in 2-fold higher ubH2B, and 2-fold lower transcriptional elongation at IRF1. We also demonstrate that USP22 depletion diminishes 3'-end cleavage/polyadenylation by 2- to 3-fold. Furthermore, the polyadenylation factor CPSF73 is not effectively recruited, and serine 2 phosphorylation (Ser2P) of the C-terminal domain of RNA polymerase II is also disrupted. The transcriptional and processing defects observed in the USP22-knockdown cells are reversed by transient USP22 overexpression. Together, these results suggest that ubH2B helps recruit polyadenylation factors to STAT1-activated genes. We propose a working model, wherein a cycle of H2B ubiquitination/deubiquitination specifies Ser2P to regulate elongation and 3'-end processing of JAK-STAT-inducible mRNAs. These results further elaborate USP22 function and its role as a putative cancer stem cell marker.