mRNA export factors store nascent transcripts within nuclear speckles as an adaptive response to transient global inhibition of transcription.

Several transcription inhibitors have been developed as cancer therapies. However, they show modest clinical activity, highlighting that our understanding of the cellular response to transcriptional inhibition remains incomplete. Here we report that potent inhibitors of transcription not only impact mRNA output but also markedly impair mRNA transcript localization and nuclear export. We demonstrate that retention of newly transcribed mRNA in nuclear speckles is an adaptive response to chemically distinct transcriptional inhibitors. Retained transcripts are fully processed and accumulate in proportion to the expression level of the genes from which they emanate. The TREX mRNA export complex plays an integral role in directing nascent transcripts to nuclear speckles where they are bound to NXF1, protected from degradation, and poised for rapid export following re-initiation of transcription. Our findings provide new insights into the crosstalk between transcription and mRNA export with important implications for drugs aiming to inhibit transcription for therapeutic gain. [Display omitted] • Global inhibition of transcription impairs mRNA localization and nuclear export • TREX directs mRNAs to the nuclear speckle where they are bound by NXF1 • Mature, export-competent mRNAs are retained and protected in nuclear speckles • Reinitiation of transcription leads to rapid mRNA export preserving cell identity Transcription inhibitors also disrupt nuclear export. Here, Williams et al. reveal that mRNA export factors sense transcription inhibition and adapt by storing mature export-competent mRNA in nuclear speckles. This enables rapid release when transcription resumes and ensures retention of cellular identity and viability during a transient global transcription insult. [ABSTRACT FROM AUTHOR]