Evolutionary analysis of a cluster of ATP-binding cassette (ABC) genes

To study the evolutionary history of ATP-binding cassette (ABC) transporters in mammals, we have characterized a cluster of five ABCA-subfamily genes localized on mouse Chromosome (Chr) 11. The genes, named Abca5, Abca6, Abca8a, Abca8b, and Abca9, are arranged in a head-to-tail fashion in a cluster that spans about 400 kb of the genomic DNA, each gene occupying about 70 kb. The transcripts of these genes contain an open reading frame from 4863 (for Abca8a and Abca8b) to 4929 (for Abca5) nucleotides, and have distinct tissue-specific expression pattern. The predicted proteins contain two transmembrane domains and two nucleotide binding domains, arranged similar to the other members of ABCA subfamily. Similarity of both the genomic organization and primary structure among the genes in this cluster suggests that the duplications generating the cluster occurred relatively recently compared with most of the ABC genes in present-day mammalian genomes. For instance, the Fugu rubripes genome contains an ortholog for only one gene, Abca5, from this cluster. Phylogenetic and comparative sequence analysis reveals that after the divergence of rodent and primate lineages, at least one gene has been lost in each genome. In addition, we found that both mouse and human clusters show evidence of a number of gene conversions, in several cases involving intron sequences.