Structure and chromosomal assignment of the sterol 12 alpha-hydroxylase gene (CYP8B1) in human and mouse: Eukaryotic cytochrome P-450 gene devoid of introns

Sterol 12 alpha-hydroxylase (CYP8B1) is a hepatic cytochrome P-450 that con trols the ratio of cholic acid over chenodeoxycholic acid in bile and thus controls the solubility of cholesterol. Both the human and the mouse CYP8B1 complementary DNA and gene were cloned and structurally characterized. Sur prisingly, the genomic DNA from both species was found to lack introns. The major transcript of the human gene was estimated to be 3950 bp, and the pu tative promoter region was estimated to be at least 1360 bp. The murine str uctural gene was found to span approximately 3 kb. By using FISH and radiat ion hybrid mapping techniques, the human CYP8B1 gene was located to chromos ome 3p21.3-p22, whereas FISH mapped the murine counterpart to chromosome 9q F4, a region that is homologous to the third human chromosome. The results from the chromosome mapping and Southern blotting indicated that the gene i s present in a single copy. Transcription of the mouse and human CYP8B1 gen es was initiated from a position situated 51 and 35 bases, respectively, do wnstream of a consensus TATA box. A homology of 21% for the promoter region s of mouse and human may indicate differences in transcriptional regulation . Although a potent induction of CYP8B1 mRNA was observed upon starvation o f mice, the mechanism behind this effect was not revealed by analysis of th e promoter for potential cis-acting elements. In the human promoter, severa l possible cis-acting regions were identified but none of them could be dir ectly related to bile acid metabolism. After transfection of COS cells with the human coding region, mRNA and enzymatic activity for the 12 alpha-hydr oxylase were identified. This is the first mammalian cytochrome P-450 gene reported to lack introns. The importance of this structural feature for evo lution and gene regulation is discussed, (C) 1999 Academic Press.