Identifizierung von cis- und transregulatorischen Elementen der 3-alpha Hydroxysteroiddehydrogenase/ Carbonylreduktase-Expression in Comamonas testosteroni

Microorganisms capable of utilizing various naturally occurring steroids, including cholesterol and the phytosterols {beta}-sitosterol, stigmasterol, and campestrol, as carbon and energy sources are relatively widespread in nature. Complete assimilation of these substrates is achieved through an adaptive complex metabolic pathway involving many enzymatic steps of oxidation responsible for the breakdown of the steroid nucleus. Since the isolation of Comamonas testosteroni from various soil samples, some work has emerged on the steroid-inducible characteristics of this organism. As a result, several steroid-metabolizing enzymes as well as steroid binding or transport activities have been described and characterized. Since the pioneering work of Talalay and co-workers, it is well known that 3{alpha}-hydroxysteroid dehydrogenase is one of the first enzymes of the steroid-catabolic pathway and therefore plays a central role in steroid metabolism. However, the exact mechanism of regulation of this important enzyme by steroids has long been unknown. In previous investigations, the gene encoding 3{alpha}-hydroxy-steroid dehydrogenase/carbonyl reductase (3{alpha}-HSD/CR)1 (hsdA) was cloned and overexpressed in Escherichia coli.Interestingly, 3{alpha}-HSD/CR from C. testosteroni has been found to be inducible by testosterone and progesteron, but the mechanism of steroid-dependent gene regulation in procaryotes remained obscure. Recently, we reported the first molecular determinant of steroid signaling in C. testosteroni. By investigating the cis- and trans-acting elements of hsdA expression in C. testosteroni, we characterized two palindromic operator domains upstream of hsdA and identified a new gene coding for a trans-acting negative regulator (repressor A, RepA) of hsdA expression.