A Micro-evolutionary Change in Target Binding Sites as a Key Determinant of Ultrabithorax Function in Drosophila

Abstract

All Hox proteins are known to recognize, in vitro, similar DNA-binding sites containing a TAAT core sequence. This poor DNA-binding specificity is in sharp contrast with their specific functions in vivo. Here we report a new binding motif with TAAAT core sequence to which the Hox protein Ultrabithorax (Ubx) binds with higher affinity and specificity. Using transgenic and luciferase assays, we show that this new motif is critical for Ubx-mediated regulation of a target gene in Drosophila melanogaster. Interestingly, this new motif with TAAAT core sequences is not associated with the targets of Ubx in the honeybee, Apis mellifera, wherein hindwings are nearly identical to the forewings. We show that introduction of TAAAT motif in the place of TAAT motif is sufficient to bring an enhancer of a wing-promoting gene of A. mellifera under the regulation of Ubx. Our results, thus, suggest that binding motifs with a TAAAT core sequence may help identify functionally relevant direct targets of Ubx in D. melanogaster and the emergence of these binding sites may be crucial for Hox-mediated morphological changes during insect evolution.