Spatial organization of transcribed eukaryotic genes

SUMMARY Despite the well-established role of nuclear organization in gene expression regulation, little is known about the reverse: how transcription shapes the spatial organization of the genome. Owing to small sizes of most previously studied genes and the limited resolution of microscopy, the structure and spatial arrangement of a single transcribed gene are still poorly understood. Here, we make use of several long highly expressed genes and demonstrate that they form transcription loops with polymerases moving along the loops and carrying nascent RNAs. Transcription loops can span across microns resembling lampbrush loops and polytene puffs. Extension and shape of transcription loops suggest their intrinsic stiffness, which we attribute to decoration with multiple voluminous nascent RNPs. Our data contradict the model of transcription factories and indicate that although microscopically resolvable transcription loops are specific for long highly expressed genes, the mechanisms underlying their formation can represent a general aspect of eukaryotic transcription.