Gene structure of a phenobarbital-inducible cytochrome P-450 in rat liver.

The gene structure of a phenobarbital-inducible rat liver cytochrome P-50 was elucidated by sequence analysis of two isolated genomic clones. The total length of the gene was approximately 14 kilobases and was separated into nine exons by eight intervening sequences. Nucleotide sequences of all exon/intron boundaries follow the G-T/A-G rule. A putative transcription initiation site was assigned to an A, 30 base pairs upstream from the initiation codon by S1 nuclease protection mapping. A possible "TATA" equivalent sequence, C-A-T-A-A-A, was found 27 base pairs further upstream from this initiation site. A poly(A) attachment site was determined to be 386 or 387 base pairs downstream from the termination codon by comparison with the cDNA sequence. Detailed comparison with two cDNA sequences determined previously showed the coding nucleotide sequence of the genomic clones to concur with that of the pcP-450pb2 cDNA clone coding for cytochrome P-450e except for three neutral base substitutions. Therefore, we conclude that the gene sequence determined here is for the cytochrome P-450e gene or a similar gene. On the other hand, 40 base substitutions were found in about 1,900 base pairs compared between the sequences of the genomic clone and the other cDNA clones (pcP-450pbl and 4) coding for cytochrome P-450b, and 15 of them result in 14 amino acid replacements in the total 491 amino acid residues. These base substitutions occur in relatively limited regions of the sequences. Most of them are found in exons 6, 7, 8, and 9; most frequently in exon 7.