GSE68686
In beta-actin knockout mouse embryonic fibroblasts epigenetic reprogramming and chromatin modifications lead to rDNA transcription inactivation and defects in cell growth and proliferation
Actin and nuclear myosin 1 (NM1) are emerging as regulators of transcription and chromatin organization. Using a genome-wide approach we report here that ß-actin binds both intergenic and genic regions across the entire mammalian genome, associated with both protein-coding and rRNA genes. Across the rDNA transcription unit the distribution of ß-actin correlated with NM1 and the other subunits of the B-WICH complex, WSTF and SNF2h. In ß-actin-/- mouse embryonic fibroblasts (MEFs), we found that rRNA synthesis levels are down-regulated concomitantly with drops in Pol I and NM1 occupancies across the rRNA gene. In knock-in experiments, reintroduction of wild-type ß-actin but not mutated forms with polymerization defects, rescued rRNA synthesis underscoring the direct role for ß-actin in Pol I transcription and the importance of polymerization during the transcription process. The rRNA defects in the ß-actin-/-MEFs are accompanied by epigenetic reprogramming that leads to up-regulation of the repressive mark H3K4me1. In addition, a high resolution chromatin accessibility assay showed that the absence of ß-actin leads to a more compact chromatin at specific locations, including promoter-proximal enhancer (T0 sequence) and Sal boxes (T1-T10), disturbing binding of the transcription termination factor 1 (TTF1). We propose a novel mechanism where the polymerase-associated ß-actin synergizes with NM1 to coordinate the establishment of permissive chromatin with the correct rDNA topology for Pol I transcription enhancement. Overall design: Association of beta-actin with the genome in mouse embryonic fibroblasts.