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.
Highlights
The autonomy of segment-specific regulatory domains in the Bithorax complex is conferred by boundary elements and associated Polycomb response elements (PREs)
A second ~ 2 kb deletion, Fab-63, extends from a site in iab-5 ~ 1.1 kb from hypersensitive region 1 (HS1) to a site within HS1 located just beyond the CTCF sites (Fig. 1b). Both deletions give a mixture of GOF/LOF phenotypes in A5 (PS10) and A6 (PS11)
A third ~ 4,7 kb iab-6Δ7 deletion, that removes HS2, had no apparent phenotypic effects[8]. Based on these findings and those of Perez-Lluch et al.[41], it was proposed that HS1 is the core Fab-6 boundary while HS2 corresponds to the iab-6 PRE
Summary
The autonomy of segment-specific regulatory domains in the Bithorax complex is conferred by boundary elements and associated Polycomb response elements (PREs). The Drosophila Bithorax complex (BX-C) has three homeotic genes, Ultrabithorax (Ubx), abdominal-A (abd-A) and Abdominal-B (Abd-B), the pattern of expression of which in the posterior parasegments/segments, PS5-14/ T3-A9 determines their identity[1,2,3] This expression pattern is generated by a unique collection of tissue specific enhancers that are located in nine functionally autonomous cis-regulatory domains. Pair-rule and maternal gene products disappear during gastrulation, regulation of BX-C switches to the maintenance phase In this phase the on and off states of the regulatory domains are maintained by Trithorax (Trx) and Polycomb (PcG) group proteins, respectively[3,9].
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