Tox4 is an evolutionarily conserved regulator of elongation and recycling of RNA polymerase II (scRNA-seq)

HMG-box protein Tox4 is a regulator of PP1 phosphatases with unknown function in development. Here we show that although pan-hematopoietic Tox4 knockout produces minimal defects, T cell-specific knockout reduces thymic cellularity by 50% and increases cell death. In addition, single cell RNA-seq discovered that Tox4 loss impairs T cell development starting from double positive (DP) blast stage partially due to downregulation of genes critical for cell cycle progression through G2 and M phases. Moreover, Tox4 loss decreases transcriptional output of thousands of genes in DP cells, and mechanistically, Tox4 restricts transcriptional elongation and facilitates transcriptional reinitiation in a dephosphorylation-dependent manner, which is consistent with what we have found in human cells. Furthermore, small effect of Tox4 loss on chromatin accessibility suggest that it mainly regulates transcription in an HMG box independent manner. These results establish an evolutionarily conserved a role of Tox4 in transcriptional elongation and reinitiation. Overall design: scRNA-seq expriments of thymocytes purified from Tox4 WT and cKO mice