Regulation of the Steroid-inducible 3α-Hydroxysteroid Dehydrogenase/Carbonyl Reductase Gene in Comamonas testosteroni

The Comamonas testosteroni3α-hydroxysteroid dehydrogenase/carbonyl reductase gene (hsdA) codes for an adaptive enzyme in the degradation of steroid compounds. However, no information was available on the molecular regulation of steroid-inducible genes nor on the mechanism of steroid signaling in procaryotes. We, therefore, investigated thecis- and trans-acting elements ofhsdA expression to infer the mechanism of its molecular regulation by steroids. The gene was localized on a 5.257-kilobase EcoRI fragment of C. testosteroni chromosomal DNA. The promoter was characterized, and the transcriptional start site was identified. Two palindromic operator domains were found upstream of hsdA. A new gene coding for a trans-acting negative regulator (repressor A, RepA) ofhsdA expression was characterized. The specific interaction between RepA, testosterone, and the operator domain is demonstrated. From our results we conclude that hsdA is under negative transcriptional control by an adjacent gene product (RepA). Accordingly, induction of hsdA by steroids in fact is a derepression, where steroidal inducers bind to the repressor, thereby preventing its binding to the hsdA operator. AF092031