A Constitutive Member of the Rat Cytochrome P450IIB Subfamily: Full-Length Coding Sequence of the P450IIB3 cDNA

The prototypic members of the rat liver cytochrome P450IIB subfamily, P450b and P450e, have long been the subjects of intense interest, in part because they are highly inducible by phenobarbital (PB). We have previously cloned and sequenced an 858-bp cDNA fragment (the PB24 insert) that encodes the carboxy-terminal portion of a P450b/P450e-like protein, henceforth referred to as P450IIB3. A Bam Hl-Eco RI fragment of the PB24 insert hybridizes with a 1.9-kb mRNA present constitutively in rat liver and not inducible by PB (Affolter et al., 1986). We have now obtained, from a λgt11 rat liver cDNA library, cDNA inserts corresponding to the complete coding sequence of the IIB3 mRNA. The 491-amino acid IIB3 protein sequence, deduced from the cDNA sequence, is 77% identical to that of P450b. Northern blot analysis, with a IIB3-specific oligonucleotide probe, confirmed the constitutive presence of the polyadenylated 1.9-kb IIB3 mRNA in male rat liver. The IIB3 mRNA was undetectable in the lung, kidney, and prostate. It was constitutively present, and not inducible by PB, in female rat liver. Our results demonstrate unequivocally the existence of a constitutive member of the rat P450IIB subfamily. The remarkable inducibility of the P450b/ P450e genes is in striking contrast to the absence of a PB effect on IIB3 gene expression. The molecular basis for this difference remains to be revealed.