| Summary: | <p>Abstract</p> <p>Background</p> <p>We have previously identified associations between major histocompatibility complex (MHC) class I and resistance towards bacterial and viral pathogens in Atlantic salmon. To evaluate if only MHC or also closely linked genes contributed to the observed resistance we ventured into sequencing of the duplicated MHC class I regions of Atlantic salmon.</p> <p>Results</p> <p>Nine BACs covering more than 500 kb of the two duplicated MHC class I regions of Atlantic salmon were sequenced and the gene organizations characterized. Both regions contained the proteasome components PSMB8, PSMB9, PSMB9-like and PSMB10 in addition to the transporter for antigen processing TAP2, as well as genes for KIFC1, ZBTB22, DAXX, TAPBP, BRD2, COL11A2, RXRB and SLC39A7. The IA region contained the recently reported MHC class I <it>Sasa-ULA </it>locus residing approximately 50 kb upstream of the major <it>Sasa-UBA </it>locus. The duplicated class IB region contained an MHC class I locus resembling the rainbow trout <it>UCA </it>locus, but although transcribed it was a pseudogene. No other MHC class I-like genes were detected in the two duplicated regions. Two allelic BACs spanning the <it>UBA </it>locus had 99.2% identity over 125 kb, while the IA region showed 82.5% identity over 136 kb to the IB region. The Atlantic salmon IB region had an insert of 220 kb in comparison to the IA region containing three chitin synthase genes.</p> <p>Conclusion</p> <p>We have characterized the gene organization of more than 500 kb of the two duplicated MHC class I regions in Atlantic salmon. Although Atlantic salmon and rainbow trout are closely related, the gene organization of their IB region has undergone extensive gene rearrangements. The Atlantic salmon has only one class I <it>UCA </it>pseudogene in the IB region while trout contains the four MHC <it>UCA</it>, <it>UDA</it>, <it>UEA </it>and <it>UFA </it>class I loci. The large differences in gene content and most likely function of the salmon and trout class IB region clearly argues that sequencing of salmon will not necessarily provide information relevant for trout and vice versa.</p>
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