Mammalian multidrug-resistance gene: correlation of exon organization with structural domains and duplication of an ancestral gene.

Analysis of the nucleotide and deduced amino acid sequences of the biologically active mouse mdr1 cDNA clone indicates that the protein is formed by two highly homologous halves, each containing six putative transmembrane domains and a nucleotide-binding site. The duplicated unit shows high sequence homology to the proposed energy-coupling subunit of bacterial periplasmic transport proteins. We have cloned and characterized the mouse mdr1 gene and have analyzed the genomic organization of the two homologous halves forming the mdr1 protein. The gene spans 68 kilobases, is split into 28 exons, and the two homologous halves are encoded by 14 and 13 exons. The transcriptional initiation site of the gene has been mapped and putative TATA and consensus CAAT sequences have been found at positions -27 and -83, respectively. Discrete structural domains of the mdr1 protein are encoded by separate exons: Ten of the 12 putative transmembrane domains are encoded by individual exons and the two nucleotide-binding sites are each encoded by three exons. The exon/intron organization of the gene is conserved in the two highly homologous regions encoding the nucleotide-binding sites. The conservation of certain pairs of introns, together with the high degree of sequence homology, indicate that the mouse mdr1 gene originated from the duplication of an intron-containing ancestral gene.