GATA-4, -5, and -6 zinc finger and hepatocyte nuclear factor-1␣ (HNF-1␣) homeodomain transcription factors are expressed in the intestinal epithelium and synergistically activate the promoter of intestinal genes. Here, we demonstrate that GATA-5 and HNF-1␣ physically associate both in vivo and in vitro and that this interaction is necessary for cooperative activation of the lactasephlorizin hydrolase promoter. Furthermore, physical association is mediated by the C-terminal zinc finger of GATA factors and the homeodomain of HNF-1␣. Deletion of HNF-1␣ activation domains or interruption of HNF-1-binding sites in the lactase-phlorizin hydrolase promoter resulted in a complete loss of cooperativity, whereas deletion of GATA-5 activation domains or interruption of GATA-binding sites resulted in a reduction, but not an elimination, of cooperativity. We hypothesize that GATA/HNF-1␣ cooperativity is mediated by HNF-1␣ through its activation domains, which are oriented for high levels of activation through binding to DNA and physical association with GATA factors. These data suggest a paradigm whereby intestine-specific gene expression is regulated by unique interactions among tissuerestricted transcription factors coexpressed in the intestine. Parallel mechanisms in other tissues as well as in Drosophila suggest that zinc finger/homeodomain interactions are an efficient pathway of cooperative activation of gene transcription that has been conserved throughout evolution.The intestinal epithelium is a dynamic structure that undergoes a highly regulated process of cell division, migration, cell fate determination, and differentiation (1-3). During intestinal development, interactions between visceral endoderm and mesoderm at E8 1 in mice result in the formation of a primitive foregut that rapidly undergoes cytodifferentiation, so that by E19, an epithelial monolayer overlies nascent villi. During the first 2 weeks of postnatal life, a proliferating compartment develops into the crypts of Lieberkü hn. Stem cells located near the base of crypts rapidly divide and give rise to four terminally differentiated cell types, which migrate both basally and apically. Cells migrating to the base of crypts become Paneth's cells, whereas those migrating up the crypt toward villi become absorptive enterocytes, goblet cells, and enteroendocrine cells. At the crypt-villus junction, the proliferative phase ends, and cells acquire a differentiated phenotype characterized by the synthesis of functionally relevant proteins. The cells continue to migrate up the villi, enter an apoptotic cycle, and are shed into the intestinal lumen ϳ3 days after their initial appearance on villi. The molecular mechanisms underlying the dynamic processes of intestine-specific gene expression and cellular differentiation during development are poorly understood.Absorptive enterocytes compose ϳ95% of epithelial cells on villi and are the cells responsible for the terminal digestion and absorption of nutrients. Lactase-phlorizin hydrolase (LPH), the enzyme critical ...