Improving the biotransformation of phytosterols to 9α-hydroxy-4-androstene-3,17-dione by deleting embC associated with the assembly of cell envelope in Mycobacterium neoaurum

•The embC involved in the lipoarabinomannan synthesis in M. neoaurum was located.•The assembly defect of lipoarabinomannan enhanced cell permeability and steroid uptake.•The deficiency of embC improved the bioconversion of sterols to 9-OHAD. The conversion of low value-added phytosterols into 9α-hydroxy-4-androstene-3,17-dione (9-OHAD) by mycobacteria is an important step in the steroid pharmaceutical industry. However, the highly dense cell envelope with extremely low permeability largely affects the overall transformation efficiency. Here, we preliminarily located the key gene embC required for the synthesis of lipoarabinomannan from lipomannan in Mycobacterium neoaurum. The genetic manipulation of embC indicated that it might be the only functional enzyme catalyzing the above synthesis process. The deficiency of lipoarabinomannan led to a significantly increased cell permeability, which in turn caused the enhanced uptake capacity of cells. The sterol substrate conversion efficiency of mycobacterial cells was increased by about 52.4 % after 72-h conversion. Ultimately, the absence of embC increased the productivity from 0.0927 g/L/h to 0.1031 g/L/h, as confirmed by a resting cell system. This study verified the feasibility of improving the efficiency of the microbial conversion system through the cell envelope engineering strategy.