GENE STRUCTURE AND FUNCTIONAL-ANALYSIS OF THE HUMAN NA+ PHOSPHATE COTRANSPORTER/

Three lambda phage clones encompassing the Na+/phosphate cotransporter (NaPi-3) gene and its 5' flanking region were isolated from a human g enomic DNA library. The gene comprises 13 exons and 12 introns and spa ns approx. 14 kb. All exon-intron junctions conform to the GT/AG rule. The major transcription-initiation site was determined by primer-exte nsion analysis and is an adenosine residue 57 bp upstream of the 3' en d of the first exon. There is a typical TATA box 28 bp upstream of the major transcription-initiation site and various cis-acting elements, including a cAMP-responsive element, AP-1, AP-2 and SP-1 sites in the 5' flanking region. This region also contains three direct repeat-like sequences that resemble the consensus binding sequence for members of the steroid-thyroid hormone receptor superfamily, including vitamin D . Deletion analysis suggests that the region from nt -- 2409 to nt -- 1259 in the 5' flanking region may be involved in kidney-specific gene expression. Vitamin D responsiveness of the NaPi-3 promoter was also detected in COS-7 cells co-transfected with a human vitamin D receptor expression vector. The presence of the three vitamin D receptor-respo nsive elements in the NaPi-3 promoter may be important in mediating th e enhanced expression of the gene by 1,25-dihydroxyvitamin D-3.