The earliest response of most organisms to carcinogenic or toxic aromatic hydrocarbons is the induction of a set genes which are involved in the metabolism the carcinogen. Paradoxically, these enzymes activate the toxic or carcinogenic potential of the inducing hydrocarbon. The cytochromes P1-450 and P3-450 of mouse or P450c and P450d of rat, are the major induced proteins in this response in rodents, although induction of other proteins can also be observed. The regulation of both cytochromes has been shown to be transcriptional although the molecular basis of this regulation has only been studied for P1-450. The goal of this proposal, is to study the mechanism of P3-450 gene regulation in a cultured primary rat hepatocyte cell culture system which we have recently shown can express P450d in response to hydrocarbon. We plan to define the cisacting DNA elements required for MCA induction of the P3-450 gene, as well as to identify transacting proteins which can interact with the regulator DNA elements to bring about transcriptional activation. We further intend to develop cloned probes for the analysis of other non-P450 genes which are induced by MCA. With such cloned probes, the molecular basis of their induction can also be determined. By comparing a set of hydrocarbon coregulated genes, the mechanisms of gene regulation may be determined.