Fountains are heterogeneous. (A) Hi-C snapshot of a locus containing broadly distributed PQN-85 around fountain origins forming a TAD-like pattern. The trajectory cartoon shows one possible interpretation in which broadly loaded cohesins form multilayered nested loops (blue, green, purple), giving rise to a broad region of enriched contact. (B) Hi-C snapshot of a locus containing two nearby fountains forming “stripes.” The trajectory cartoon shows one possible interpretation, in which collision of two-sided loop-extruding cohesins from two fountain origins stall each other, causing them to behave as one-sided loop-extruding factors (red). The collision results in nested loops, in which the outer loop (blue) forms enriched contact away from the main diagonal.
