INTESTINAL EXPRESSION OF THE CALBINDIN-D9K GENE IN TRANSGENIC MICE - REQUIREMENT FOR A CDX2-BINDING SITE IN A DISTAL ACTIVATOR REGION

The calbindin-D9K gene encodes a vitamin D-induced calcium-binding pro tein that is expressed as a marker of small intestine differentiation. We have shown that 4580 base pairs of its 5' DNA regulatory region ca n target reporter transgene expression in the intestine and cause this transgene to respond like the endogenous gene to vitamin D active met abolite and that the homeoprotein Cdx2 is bound to the TATA box in the intestine. We now show that the 4580 base pairs construct confers a d ifferentiated pattern of reporter transgene expression in the intestin e and that cooperation between the proximal promoter and a distal elem ent located in an opened chromatin structure is responsible for the in testinal expression and vitamin D responsiveness of the transgene, Gel shift and footprinting assays using duodenal nuclear extracts indicat e that this distal element contains a Cdx2-binding site. Finally, a mu tation in this distal Cdx2-binding site dramatically decreases intesti nal expression in transgenic mice. This report, using an in vivo appro ach, demonstrates the crucial role of Cdx2 for the transcription of an intestinal gene.