Regulatory architecture of the Drosophila IAB7b enhancer. Lauren N. Winkler¹, Jessica S. Kurata¹, Michael J. Nevarez¹, Lily Li¹, Jacqueline M. Dresch², Robert A. Drewell¹. 1) Biology Department, Harvey Mudd College, Claremont, CA; 2) Mathematics Department, Harvey Mudd College, Claremont, CA.

   In Drosophila, cellular identity along the antero-posterior axis falls under the control of two homeotic (Hox) gene complexes. The 330 kb bithorax complex (BX-C), which regulates cell type differentiation during development in the posterior thorax and abdomen, comprises three Hox genes: Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B). The expression of each of these genes is in turn controlled through interactions between transcription factors (TFs) and a number of cis-regulatory modules (CRMs) in the neighboring intergenic regions. We sought to determine how the sequence architecture of TF binding sites mediates the functional activity of a CRM.
   To address this issue, we are investigating the IAB7b enhancer, which regulates Abd-B expression in the seventh abdominal segment (A7/PS12) of the early Drosophila embryo. A cross-species comparison of the IAB7b enhancer reveals a conserved motif containing two FUSHI-TARAZU (FTZ) TF binding sites in close proximity to two KRUPPEL (KR) binding sites. This signature motif is capable of driving reporter gene expression in A5, A7, and A9. We investigated the ability of the transcriptional repressor KNIRPS (KNI) to suppress reporter gene expression in A5. Additionally, we examined the functional importance of the spacing between the two FTZ binding sites using both computational and molecular genetic experimental approaches. Our studies demonstrate that the transcriptional output of the IAB7b CRM relies on a complex set of combinatorial inputs mediated by TF binding.