DDX3X acts as a selective dual switch regulator of mRNA translation in acute ER stress.

Regulation of eukaryotic mRNA translation initiation greatly impacts gene expression and is critical for cellular stress response. DDX3X is a ubiquitous DEAD-box RNA helicase whose precise role in scanning and translation regulation in non-stressed and stressed cells remains incompletely understood. Here, we show that DDX3X associates with thousands of mRNAs as part of the eIF4F-mediated 48S scanning complex and exerts dual regulatory effects, promoting or repressing translation of select mRNAs under basal conditions and reversing this regulation during acute endoplasmic reticulum stress. Initiation profiling reveals mechanistically distinct modes of DDX3X action linked to its binding patterns across the 5' UTR and coding sequence. We further uncover that mRNAs selectively regulated by DDX3X exhibit specific patterns of cytidine N4-acetylation near start codons, with shared de-repression observed upon NAT10 knockdown. Together, our findings reveal DDX3X as a context-sensitive regulator that has a possible functional connection with epitranscriptomic features in translation control.