Cytochrome P1-450 Structural Gene in Mouse, Rat, and Rabbit: Differences in DNA Methylation and Developmental Expression of mRNA*

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Abstract
Clone 46 previously was shown to represent an 1100-bp cDNA clone of the mouse cytochrome P1-450 structural gene. Clone 46 [32P]DNA was hybridized to DNA and mRNA from mouse, rat, and rabbit of different ages. In Hpa II digests of DNA, two hybridizable fragments of less than 0.5 kb exist in adult "Ah-responsive" C57BL/6N mouse liver but not in C57BL/6N sperm or embryo or in adult "Ah-nonresponsive" DBA/2N mouse liver. The reason for this hypomethylation of adult C57BL/6N liver DNA is not known but might be related to the high degree of expressivity of this gene in adult C57BL/6N liver, compared with adult DBA/2N liver. No differences in Hpa II- or Msp I-digested DNA are seen in C57BL/6N or DBA/2N inbred strains treated with the P1-450 inducer, 3-methylcholanthrene (3-MC), versus untreated controls. The ontogenetic expression of 3-MC-induced P1-450 mRNA (23 S) from mouse or rat liver corresponds well to previous developmental studies from this laboratory involving 3-MC-inducible aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) (EC 1.14.14.1) activity. P1-450 mRNA—induced transplacentally by 3-MC given to the mother—is readily detectable by clone 46 as early as gestational day 15. The cloned cDNA probe hybridizes to rat and rabbit DNA fragments of different sizes, and with less intensity, when compared with hybridization to mouse DNA. No hybridization of this DNA is observed with rabbit mRNA of all ages ranging from neonate to adult. These data suggest that sequence homology exists among the mouse, rat, and rabbit P1-450 structural genes and between mouse and rat P1-450 mRNA. The mouse cDNA probe is believed to hybridize to a segment of the rabbit P1-450 gene that is not transcribed.