Cloning, overexpression, purification, crystallization and preliminary X-ray analysis of 3-­ketosteroid Δ⁴-(5α)-dehydrogenase from Rhodococcus jostii RHA1

3-Ketosteroid dehydrogenases are flavoproteins which play key roles in steroid ring degradation. The enzymes are abundantly present in actinobacteria, including the catabolic powerhouse Rhodococcus jostii and the pathogenic species R. equi and Mycobacterium tuberculosis. The gene for 3-ketosteroid Δ⁴-(5α)-dehydrogenase [Δ⁴-(5α)-KSTD] from R. jostii RHA1 was cloned and overexpressed in Escherichia coli. His-tagged Δ⁴-(5α)-KSTD enzyme was purified by Ni²⁺–NTA affinity chromatography, anion-exchange chromatography and size-exclusion chromatography and was crystallized using the hanging-drop vapour-diffusion method. Seeding greatly improved the number of crystals obtained. The crystals belonged to space group C222₁, with unit-cell parameters a = 99.2, b = 114.3, c = 110.2 Å. Data were collected to a resolution of 1.6 Å.