The Late Boundary Complex (LBC) Selectively Binds the Fab-7/Fab-8 Boundaries, but not Fab-6 or Mcp, in the Abd-B Regulatory Region of Drosophila melanogaster

Boundary elements in the Drosophila Bithorax complex partition the Abd-B regulatory landscape into autonomous domains that require insulation and boundary bypass. We surveyed Large Boundary Complex (LBC) occupancy across Abd-B boundaries and detected binding at Fab-7 and Fab-8, but not at Fab-6 or at Mcp (the boundary between Abd-B and abd-A). Despite lacking LBC binding, Fab-6 supports boundary bypass, demonstrating that bypass can occur without LBC. Fab-7 and Fab-8 share two conserved motifs ("green" and "blue"); however, mutational analysis shows that neither motif is required for LBC recruitment. These findings reveal selective, context-dependent LBC occupancy at Abd-B boundaries and imply that additional factor(s) and/or higher-order sequence organization underlie bypass activity. Thus, LBC is not universally required for boundary bypass, and its recruitment to Fab-7/8 is independent of the shared "green/blue" motifs.