A dual‐function SNF2 protein drives chromatid resolution and nascent transcript removal in mitosis

Mitotic chromatin is largely assumed incompatible with transcription due to changes in the transcription machinery and chromosome architecture. However, the mechanisms of mitotic transcriptional inactivation and their interplay with chromosome assembly remain largely unknown . By monitoring ongoing transcription in Drosophila early embryos, we reveal that eviction of nascent mRNAs from mitotic chromatin occurs after substantial chromosome compaction and is not promoted by condensin I. Instead, we show that the timely removal of transcripts from mitotic chromatin is driven by the SNF2 helicase‐like protein Lodestar (Lds), identified here as a modulator of sister chromatid cohesion defects. In addition to the eviction of nascent transcripts, we uncover that Lds cooperates with Topoisomerase 2 to ensure efficient sister chromatid resolution and mitotic fidelity. We conclude that the removal of nascent transcripts upon mitotic entry is not a passive consequence of cell cycle progression and/or chromosome compaction but occurs via dedicated mechanisms with functional parallelisms to sister chromatid resolution. Synopsis The SNF‐2 helicase‐like protein Lodestar (Lds) functions in two pathways essential for mitotic fidelity: timely removal of nascent transcripts from mitotic chromatin and efficient sister chromatid resolution. Loss of Lds strongly suppresses the defects associated with premature loss of cohesion. Live‐cell imaging of ongoing transcription reveals the kinetics of nascent transcript eviction during mitosis. Absence of Lds leads to retained mRNAs on mitotic chromosomes. In addition to the removal of nascent transcripts, Lds also promotes efficient sister chromatid resolution. Graphical Abstract The SNF‐2 helicase‐like protein Lodestar (Lds) functions in two pathways essential for mitotic fidelity: timely removal of nascent transcripts from mitotic chromatin and efficient sister chromatid resolution.