The Hox protein family consists of homeodomain-containing transcription factors that are primary determinants of cell fate during animal development. Specific Hox function appears to rely on protein-protein interactions; however, the partners involved in these interactions and their function are largely unknown. Disconnected Interacting Protein 1 (DIP1) was isolated in a yeast two-hybrid screen of a 0 -12-h Drosophila embryo library designed to identify proteins that interact with Ultrabithorax

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... (Ubx), a Drosophila Hox protein. The Ubx⅐DIP1 physical interaction was confirmed using phage display, immunoprecipitation, pull-down assays, and gel retardation analysis. Ectopic expression of DIP1 in wing and haltere imaginal discs malforms the adult structures and enhances a decreased Ubx expression phenotype, establishing a genetic interaction. Ubx can generate a ternary complex by simultaneously binding its target DNA and DIP1. A large region of Ubx, including the repression domain, is required for interaction with DIP1. These more variable sequences may be key to the differential Hox function observed in vivo. The Ubx⅐DIP1 interaction prevents transcriptional activation by Ubx in a modified yeast one-hybrid assay, suggesting that DIP1 may modulate transcriptional regulation by Ubx. The DIP1 sequence contains two dsRNAbinding domains, and DIP1 binds double-stranded RNA with a 1000-fold higher affinity than either singlestranded RNA or double-stranded DNA. The strong interaction of Ubx with an RNA-binding protein suggests a wider range of proteins may influence Ubx function than previously appreciated.