Mapping of functional elements of the Fab-6 boundary involved in the regulation of the Abd-B hox gene in Drosophila melanogaster

Abstract

The autonomy of segment-specific regulatory domains in the Bithorax complex is conferred by boundary elements and associated Polycomb response elements (PREs). The Fab‑6 boundary is located at the junction of the iab‑5 and iab‑6 domains. Previous studies mapped it to a nuclease hypersensitive region 1 (HS1), while the iab‑6 PRE was mapped to a second hypersensitive region HS2 nearly 3 kb away. To analyze the role of HS1 and HS2 in boundary we generated deletions of HS1 or HS1 + HS2 that have attP site for boundary replacement experiments. The 1389 bp HS1 deletion can be rescued by a 529 bp core Fab‑6 sequence that includes two CTCF sites. However, Fab‑6 HS1 cannot rescue the HS1 + HS2 deletion or substitute for another BX-C boundary – Fab‑7. For this it must be combined with a PRE, either Fab‑7 HS3, or Fab‑6 HS2. These findings suggest that the boundary function of Fab‑6 HS1 must be bolstered by a second element that has PRE activity.