SUMMARY Despite extensive mapping of three-dimensional (3D) chromatin structures, the basic principles underlying genome folding remain unknown. Here, we report a fundamental role for L1 and B1 retrotransposons in shaping the macroscopic 3D genome structure. Homotypic clustering of B1 and L1 repeats in the nuclear interior or at the nuclear and nucleolar peripheries, respectively, segregates the genome into mutually exclusive nuclear compartments. This spatial segregation of L1 and B1 is conserved in mouse and human cells, and occurs dynamically during establishment of the 3D chromatin structure in early embryogenesis and the cell cycle. Depletion of L1 transcripts drastically disrupts the spatial distributions of L1- and B1-rich compartments. L1 transcripts are strongly associated with L1 DNA sequences and induce phase separation of the heterochromatin protein HP1α. Our results suggest that genomic repeats act as the blueprint of chromatin macrostructure, thus explaining the conserved higher-order structure of chromatin across mammalian cells.

L1 and B1 repeats blueprint the spatial organization of chromatin