Structural study of the porcine Na+/H+ exchanger NHE1 gene and its 5'-flanking region.

The Na+/H+ exchanger is an ubiquitous mammalian plasma membrane protein that is important for the regulation of intracellular pH and cell volume. In order to provide some insight into the molecular basis of NHE1 expression we have isolated and characterized genomic DNA clones containing the coding region and 5'-flanking region of the porcine NHE1 gene. The gene spans more than 30 kb in length and consists of twelve exons that are flanked by typical splice donor and acceptor sequences at the exon-intron boundaries. The positions of the splicing sites are conserved in relation to the human NHE1 gene. The 5' distal transcription initiation site, identified by primer extension analysis, is positioned 766 bp upstream of the translation initiation codon and 36 bp downstream of a TATA box. A 5'-flanking region of 1.62 kb in length contains a number of potential regulatory elements, and exhibits several features that distinguish the pig gene from those of rabbit, mouse and human. The NHE1 gene is located in a CpG island. The promoter sequence of 500 bp is compared with that for NHE1 genes from different species. The homology between the porcine and the human, rabbit and mouse genes is 78, 76 and 75%, respectively. Several consensus elements for transcription factors, including AP-1, C/EBP, and Sp1 are phylogenetically conserved between pig and human, while AP3 and PEA3 are found only in pig. Some conserved elements are found in the pig in multiple copies. These results suggest broadly similar regulatory mechanisms for NHE1 transcription among the different mammalian species but show some species or tissue-specific differences.