Determinants of chromosome organization: Loop extrusion or boundary:boundary pairing?

Two different models have been proposed to explain how the endpoints of chromatin looped domains (“TADs”) in eukaryotic chromosomes are determined. In the first a cohesin complex extrudes a loop until it encounters a boundary element roadblock, generating a stem-loop (and an unanchored loop). In this model, boundaries are functionally autonomous: they have an intrinsic ability to halt the movement of incoming cohesin complexes that is independent of the properties of neighboring boundaries. In the second, loops are generated by boundary:boundary pairing. In this model, boundaries are functionally non-autonomous, and their ability to form a loop depends upon how well they match with their neighbors. Moreover, unlike the loop-extrusion model, pairing interactions can generate both stem-loops and circle-loops. We have used a combination of Micro-C to analyze how TADs are organized and experimental manipulations of the even-skipped TAD boundary, homie, to test the predictions of the “loop-extrusion” and the “boundary-pairing” models. Our findings are incompatible with the loop-extrusion model and instead suggest that endpoints of TADs in flies are determined by a mechanism in which boundary elements physically pair with their neighbors, either head-to-head, or head-to-tail. Homie transgenic cassettes have been placed at an attP target site -142kb upstream of eve. Micro-C experiments were done in biological duplicates.