Characterization and engineering of 3-ketosteroid 9α-hydroxylases in Mycobacterium neoaurum ATCC 25795 for the development of androst-1,4-diene-3,17-dione and 9α-hydroxy-androst-4-ene-3,17-dione--producing strains.

3-Ketosteroid 9α-hydroxylase (Ksh) consists of a terminal oxygenase (KshA) and a ferredoxin reductase and is indispensable in the cleavage of steroid nucleus in microorganisms. The activities of Kshs are crucial factors in determining the yield and distribution of products in the biotechnological transformation of sterols in industrial applications. In this study, two KshA homologues, KshA1N and KshA2N , were characterized and further engineered in a sterol-digesting strain, Mycobacterium neoaurum ATCC 25795 to construct androstenone-producing strains. Thereinto, kshA1N is a member of the gene cluster encoding sterol catabolism enzymes, and its transcription exhibited a 4.7-fold increase under cholesterol induction. Furthermore, null mutation of kshA1N led to the stable accumulation of androst-4-ene-3,17-dione (AD) and androst-1,4-diene-3,17-dione (ADD). We determined kshA2N to be a redundant form of kshA1N Through a combined modification of kshA1N , kshA2N , and other key genes involved in the metabolism of sterols, we constructed a high-yield ADD-producing strain that could produce 9.36 g l-1 ADD from the transformation of 20 g l-1 phytosterols in 168 h. Moreover, we improved a previously established 9α-hydroxy-AD-producing strain via the overexpression of a mutant KshA1N that had enhanced Ksh activity. Genetic engineering allowed the new strain to produce 11.7 g l-1 9α-hydroxy-4-androstene-3,17-dione (9-OHAD) from the transformation of 20.0 g l-1 phytosterol in 120 h. Importance Steroidal drugs are widely used for anti-inflammation, anti-tumor, endocrine regulation, fertility management, among other uses. The two main starting materials for the industrial synthesis of steroid drugs are phytosterol and diosgenin. The phytosterol processing is carried out by microbial transformation, which is thought to be superior to the diosgenin processing by chemical conversions, given its simple and environmentally friendly process. However, diosgenin has long been used as the primary starting material instead of phytosterol. This is in response to challenges in developing efficient microbial strains for industrial phytosterol transformation, which stems from complex metabolic processes that feature many currently unclear details. In this study, we identified two oxygenase homologues of 3-ketosteroid-9α-hydroxylase, KshA1N and KshA2N , in M. neoaurum and demonstrated their crucial role in determining the yield and variety of products from phytosterol transformation. This work has practical value in developing industrial strains for phytosterol biotransformation.