Gene transcription and chromatin packing domains form a self- organizing system.

The human genome organizes into several thousand chromatin packing domains that couple euchromatin and heterochromatin into unified nanoscale volumes. Prior work suggested these domains form by transcriptionally driven loops that guide packing domain assembly. Here, we study the process of transcriptionally driven domain formation and maintenance. By pairing auxin-inducible degron technology with nanoscopic imaging, transcriptomics, and Hi-C, we show that Pol-II regulates conformationally defined interphase chromatin packing domains. Pol-II facilitates nascent domain generation and maintains mature domain integrity through the process of generating transcriptional loops. Mechanistically, Pol-II maintains the packing of intronic and intergenic chromatin within domains by transcribing exons within gene bodies. Consequently, polymerase loss disrupts genome connectivity, in situ packing domains, and gene expression, genome-wide. Our findings suggest chromatin packing domains and RNA synthesis are tightly coupled to optimize transcriptional responses in human cells.