Regulatory Elements Controlling the Basal and Drug-Inducible Expression of Glutathione S-Transferase Ya Subunit Gene

Abstract

The synthesis of the glutathione S-transferase Ya subunit is induced in the mammalian liver by chemicals such as phenobarbital and 3-methylcholanthrene. To study the mechanism of this induction, the 5′-flanking region of a mouse glutathione S-transferase Ya subunit gene was fused to the structural gene for chloramphenicol acetyltransferase. The fusion gene was introduced into hepatoma cells for the assay of the expressed acetyltransferase activity. At least two cis-regulatory elements were identified in the 5′-flanking region of the Ya gene: one, responsible for the basal level of expression, is present in the sequence up to −0.2 kb; another, responsible for the inducible expression by aromatic compounds such as β-naphthoflavone and 3-methylcholanthrene, is located in the sequence from −0.2 kb to −1.6 kb. The inducible element was functional only in cells with normal aromatic compound receptors, and it retained responsiveness to β-naphthoflavone when transfected into homologous (mouse) or heterologous (rat, human) hepatoma cells. A 150-bp region upstream from the transcription initiation site of the mouse Ya gene was investigated for cis-acting transcriptional elements that are recognized by specific DNA-binding proteins. We show by DNase I foot-printing assays using extracts from liver nuclei that the Ya gene promoter contains, in addition to the TATA and CCAAT boxes, a more distal element that binds a protein which is probably related to the family of nuclear factor 1 (NF1).