Long interspersed nuclear element 1 and B1/Alu repeats blueprint genome compartmentalization.

The organization of the genome into euchromatin and heterochromatin has been known for almost 100 years [1]. More than 50% of mammalian genomes contain repetitive sequences [2,3]. Recently, a functional link between the genome and its folding has been identified [4,5]. Homotypic clustering of long interspersed nuclear element 1 (LINE1 or L1) and B1/Alu retrotransposons forms grossly exclusive nuclear domains that characterize and predict heterochromatin and euchromatin, respectively. The spatial segregation of L1 and B1/Alu-rich compartments is conserved in mammalian cells and can be rebuilt during the cell cycle and established de novo in early embryogenesis. Inhibition of L1 RNA drastically weakened homotypic repeat contacts and compartmental segregation, indicating that L1 plays a more significant role than just being a compartmental marker. This simple and inclusive genetic coding model of L1 and B1/Alu in shaping the macroscopic structure of the genome provides a plausible explanation for the remarkable conservation and robustness of its folding in mammalian cells. It also proposes a conserved core structure on which subsequent dynamic regulation takes place.