RNA helicase DDX21 mediates nucleotide stress responses

The stress of nucleotide pool reduction regulates transcription in neural crest and melanoma cells. To better understand the molecular response caused by nucleotide stress, we designed a chemical suppressor screen for leflunomide, an inhibitor of dihydroorate dehydrogenase. We found that alterations in the progesterone receptor (Pgr) activity suppressed the neural crest effects of leflunomide. To clarify the mechanism of action, we found that the RNA helicase protein, Ddx21, binds to Pgr, and loss of function of Ddx21 conferred resistance to nucleotide stress in zebrafish embryos. At the molecular level, nucleotide stress reduces DDX21 chromatin occupancy and thus, target gene expression. Together our results show that DDX21 is a transcriptional sensor and mediator of the nucleotide stress response.