Two small domains within the Hox protein Ultrabithorax are necessary for specific DNA binding and transcripitonal repression of Dll. B.A. Gebelein , J. Culi , W. Zhang , R.S. Mann. Dept Biochemistry, Columbia Univ, New York, NY.

   Drosophila melanogaster contains eight different Hox genes that are expressed in unique patterns along the anterior-posterior axis to specify distinct segmental identities. All of the Hox genes encode transcription factors that bind DNA through a highly conserved homeodomain to regulate the expression of target genes. This finding has led to the hypothesis that Hox proteins control morphogenesis by regulating the expression of unique sets of target genes. However, most studies indicate that the majority of Hox proteins bind DNA with similar affinities and selectivities, and thus it is unclear how these proteins control distinct cell fates. In this study, we have analyzed why the Hox target gene, Distaless (Dll), is repressed by the Hox factor Ultrabithorax (Ubx) but not by Antennapedia (Antp). Through gel shift analysis we have determined that the Hox cofactors Extradenticle and Homothorax selectively stimulate Ubx binding but not Antp binding to a site in a Dll enhancer. Using this DNA binding assay in conjunction with misexpression studies, we have identified two small domains within Ubx that are each approximately 35 amino acids long, lie outside the homeodomain, and together stimulate efficient Dll binding and repression. When these domains are inserted into Antp to generate a chimeric protein, they confer transcriptional repression of Dll. In addition, the misexpression of this chimera is sufficient to transform the head and thoracic embryonic cuticle to an abdominal identity, a Ubx-like transformation. Lastly, a Ubx protein that lacks these domains has a decreased ability to repress Dll, while retaining the ability to perform other Ubx-like properties such as dpp activation and cuticle transformation. Taken together, these studies have led to the identification of two distinct domains within Ubx that give this Hox protein its unique DNA binding and transcriptional regulatory activities.