Chromatin structure of the cytochrome P-450c gene changes following induction

The chromatin structure of cytochrome P-450c and P-450d genes, which in the liver are highly inducible by 3-methylcholanthrene, was studied in normal and carcinogen-treated rats by using a cDNA probe specific for P-450c and a genomic probe that recognizes both genes. Digestion with micrococcal nuclease revealed that the active genes are not present in the typical 200 base pair nucleosomal structure. Gene induction is associated with a rearrangement of the nuclear organization of the genes. By use of indirect end-label hybridization, three DNase I hypersensitive sites were mapped, one in the 5'-terminal region and two in the 3' region of the P-450c gene. Gene induction, by treatment with 3-methylcholanthrene, changes the location of the DNase I site present in the 5' region without affecting the sites present in the 3' region. Rat thymus chromatin does not contain these DNase I hypersensitive sites, suggesting that, in the liver, the chromatin structure is altered so as to allow tissue-specific expression of the P-450c gene. The chromatin structure of the highly inducible P-450c gene is compared to that of the P-450m gene, which is induced to a significantly smaller extent and is constitutively expressed.