Structure of Genes in the Cytochrome P-450PBc Subfamily: Conservation of Intron Locations in the Phenobarbital-Inducible Family

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Abstract
The structures of P-450 genes in the rabbit phenobarbital-inducible subfamily of P-450 genes, defined by the cytochrome P-450PBc cDNAs, have been determined by restriction mapping and nucleotide sequencing. A genomic clone corresponding to the 5′-flanking region and the first six exons of the cytochrome P-450PBc2 gene has been examined in detail, and two other clones corresponding to exons 7–9 and the 3′-flanking region of the cytochrome P-450PBc3 and cytochrome P-450(form 1) genes have been characterized less extensively. The intron sequences adjacent to the exons are typical of vertebrate nuclear genes and contain GT and AG at their 5′ and 3′ ends, respectively. The intron locations in these genes are identical to those in the genes for rat cytochrome P-450b/e, members of the other phenobarbital-inducible subfamily, but completely different from those in the 3-methylcholanthrene-inducible cytochrome P-450 genes. Repetitive sequences are present in at least three introns of the cytochrome P-450PBc2 gene, but not in exons or the 5′-flanking region. The site of transcription initiation for the cytochrome P-450PBc2 gene was determined by primed reverse transcription and S1 mapping. Although the mRNA appeared to be heterogeneous at the 5′ terminus, the most probable start site is 18 bp downstream from a canonical TATA sequence and 23 bp upstream from the initiator ATG codon. The nucleotide sequence of 800 bp of the 5′-flanking region of the cytochrome P-450PBc2 gene has been determined. This region shares little homology with the 5′-flanking region of rat cytochrome P-450b/e genes, which are 55–60% homologous in the protein-coding region with the members of the P-450PBc subfamily. The amino-terminal sequence of cytochrome P-450PBc2, which could not be determined from the cDNA clones, was derived from the gene sequence. Comparison of the amino-terminal amino acid sequence of P-450PBc2 with those of other mammalian P-450s indicates that the rat isozymes f, g, h, i, and PB1 are in the same subfamily as the rabbit isozymes form 1, form 3b or P-450PBc3, P-450PBcl, and P-450PBc2.

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