Structure and nucleotide sequence of the rat intestinal vitamin D-dependent calcium binding protein gene.

The vitamin D-dependent intestinal calcium binding protein (ICaBP, 9 kDa) is under transcriptional regulation by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], the hormonal active form of the vitamin. To study the mechanism of gene regulation by 1,25-(OH)2D3, we isolated the rat ICaBP gene by using a cDNA probe. Its nucleotide sequence revealed 3 exons separated by 2 introns within approximately 3 kilobases. The first exon represents only noncoding sequences, while the second and third encode the two calcium binding domains of the protein. The gene contains a 15-base-pair imperfect palindrome in the first intron that shows high homology to the estrogen-responsive element. This sequence may represent the vitamin D-responsive element involved in the regulation of the ICaBP gene. The second intron shows an 84-base-pair-long simple nucleotide repeat that implicates Z-DNA formation. Genomic Southern analysis shows that the rat gene is represented as a single copy.