Characterization and engineering of 3-ketosteroid-△1-dehydrogenase and 3-ketosteroid-9α-hydroxylase in Mycobacterium neoaurum ATCC 25795 to produce 9α-hydroxy-4-androstene-3,17-dione through the catabolism of sterols

3-Ketosteroid-△1-dehydrogenase (KstD) is a key enzyme involved in the microbial catabolism of sterols. Here, three homologues of KstD were characterized from Mycobacterium neoaurum ATCC 25795, showing distinct substrate preferences and transcriptional responses to steroids. Single deletion of any MN-kstD failed to result in a stable and maximum accumulation of 9-OHAD due to residual KstD activities. To develop stable 9-OHAD producers, all of these MN-KstDs were inactivated, which led to about 6.02 g l−1 of 9-OHAD from 15 g l−1 of phytosterols. However, the product was mixed with 1.55 g l−1 of AD as a major by-product. To transform AD, the oxygenase component of 3-ketosteroid-9α-hydroxylase (KSH), encoded by kshA, was overexpressed. As a result, the yield of 9-OHAD increased to 7.33 g l−1 with less than 0.31 g l−1 of AD and the selectivity of 9-OHAD production was improved to 95-97% among metabolites.