Low affinity binding site clusters confer hox specificity and regulatory robustness

JUSTIN CROCKER ; NAMIKO ABE; LUCREZIA RINALDI; ALISTAIR P. MCGREGOR; NICOLÁS FRANKEL; SHU WANG; AHMAD ALSAWADI; PHILIPPE VALENTI; SERGE PLAZA; FRANCOIS PAYRE; RICHARD S. MANN; DAVID L. STERN

CELL

CELL PRESS

Lugar: United States; Año: 2015 vol. 160 p. 191 - 203

0092-8674

In animals, Hox transcription factors define regional identity in distinct anatomical domains. How Hox genes encode this specificity is a paradox, because different Hox proteins bind with high affinity in vitro to similar DNA sequences. Here, we demonstrate that the Hox protein Ultrabithorax (Ubx) in complex with its cofactor Extradenticle (Exd) bound specifically to clusters of very low affinity sites in enhancers of the shavenbaby gene of Drosophila. These low affinity sites conferred specificity for Ubx binding in vivo, but multiple clustered sites were required for robust expression when embryos developed in variable environments. Although most individual Ubx binding sites are not evolutionarily conserved, the overall enhancer architecture-clusters of low affinity binding sites-is maintained and required for enhancer function. Natural selection therefore works at the level of the enhancer, requiring a particular density of low affinity Ubx sites to confer both specific and robust expression.