SPATIAL ORGANIZATION OF TRANSCRIBED EUKARYOTIC GENES

Despite the well established role of nuclear organization in the regulation of gene expression, little is known about the reverse: how transcription shapes the spatial organization of the genome. In particular, given the relatively small sizes of genes and the limited resolution of light microscopy, the structure and spatial arrangement of a single transcribed gene are still poorly understood. Here, we make use of several long highly expressed mammalian genes and demonstrate that they form Transcription Loops (TLs) with polymerases moving along the loops and carrying nascent RNAs that undergo co-transcriptional splicing. TLs dynamically modify their harboring loci and extend into the nuclear interior suggesting an intrinsic stiffness. Both experimental evidence and polymer modeling support the hypothesis that TL stiffness is caused by the dense decoration of transcribed genes with multiple voluminous nascent RNPs. We propose that TL formation is a universal principle of eukaryotic gene expression.