pPorcine taurochenodeoxycholic acid 6α-hydroxylase, cytochrome P450 4A21 (CYP4A21), differs from other members of the CYP4A subfamily in terms of structural features and catalytic activity. CYP4A21 participates in the formation of hyocholic acid, a species-specific primary bile acid in the pig. The iCYP4A21/i gene was investigated and found to be approx. 13 kb in size and split into 12 exons. The intron–exon organization of the iCYP4A21/i gene corresponds to that of iCYP4A/i fatty acid hydroxylase genes in other species. Comparison with a genomic segment of a pig iCYP4A/i fatty acid hydroxylase gene (iCYP4A24/i) revealed a sequence identity with iCYP4A21/i that extends beyond the exons, indicating a common origin by gene duplication. A pronounced sequence identity was found also within the proximal 5′-flanking regions, whereas the patterns of mRNA expression of iCYP4A21/i and iCYP4A/i fatty acid hydroxylases in pig liver differ. Sequence comparison aiming to elucidate the origin of the unique features of iCYP4A21/i revealed a region of decreased sequence identity from exon 6 to exon 8, strongly suggesting that gene conversion could have contributed to the evolution of iCYP4A21/i./p
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