Simple Sequence DNA Associated with Near Sequence Identity of the 3′-Flanking Regions of Rat Cytochrome P450b and P450e Genes

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Abstract
In rat liver, the two major phenobarbital (PB)-inducible cytochrome P450s, P450b (P450IIB1), and P450e (P450IIB2), are encoded by ∼2.1-kb mRNAs showing more than 97% nucleotide sequence identity. Almost half of the sequence differences are concentrated in two short divergent segments in exon 7. An additional 4.8-kb P450b/P450e RNA, inducible by Aroclor and by PB, hybridizes with a classical P450b probe and with the 3′ extension of the PB23 insert, a cloned P450b-like cDNA (Affolter et al., DNA 5, 209–218, 1986). The 4.8-kb form has now been detected in several rats under a variety of induction conditions. DNA sequencing of the 5′ portion of the PB23 insert showed it is derived from a P450b gene. The 4.8-kb RNA hybridized with an oligonucleotide probe that recognizes P450b mRNA, but did not hybridize detectably with one that recognizes P450e mRNA. The 4.8-kb RNA and the PB23 insert doubtless represent P450b RNAs polyadenylated at a downstream site. DNA sequence analysis of the PB23 3′ extension (which represents the 3′ end of the P450b gene) and of the P450e gene 3′-flanking region demonstrated that the near identity of the P450b and P450e genes extends for at least 920 bp beyond the major polyadenylation site. This region was doubtless part of the original duplication that gave rise to the P450b and P450e genes. A short divergent segment is present in the 3′-flanking region of near sequence identity; the segment is embedded in simple sequence DNA that contains a mixed alternating pyrimidine/purine tract.