Heme regulates cytochrome P-450 gene transcription elongation.

Administration of 3-methylcholanthrene (MC) to rats results in a striking increase in the transcription of cytochrome P-450 (c + d) messenger RNA with isolated nuclei, which is blocked by the simultaneous administration of cobalt chloride, an inhibitor of heme biosynthesis. Transcription of cytochrome P-450 (c + d) mRNAs with nuclei isolated from MC treated rats shows a linear increase with time of incubation, whereas it shows a progressive decrease with incubation time in the case of nuclei isolated from MC+CoCl2 treated rats. Addition of heme in vitro (10(-6)M) to the latter nuclei results in a significant counteraction of the decreased cytochrome P-450 (c + d) mRNA transcription. The inhibition in transcription rates observed in MC+CoCl2 treated rat liver nuclei is more pronounced with the seventh exon probe than with the second exon probe. Once again, in vitro heme addition can counteract the inhibition observed with both the probes. Since run off transcription with isolated nuclei represents essentially elongation of the initiated transcripts, the data obtained can be interpreted on the basis that heme regulates cytochrome P-450 gene transcription elongation.