Efficient keratinase expression via promoter engineering strategies for degradation of feather wastes.

Keratinases are promising alternatives over ordinary proteases in several industrial applications due to their unique properties compared with their counterparts in the protease categories. However, their large-scale industrial application is limited by the low expression and poor fermentation efficiency of keratinase. Here, we demonstrate that the expression level of keratinase can be improved by constructing a more efficient enzyme expression system hereby enables the highest production titer as regarding recombinant keratinase production to date. Specially, ten promoters were evaluated and the aprE promoter exhibits a significant promotion of keratinase (kerBv) titer from 165 U/mL to 2605 U/mL in Bacillus subtilis. The batch fermentation mode resulted in a maximum keratinase activity of 7176 U/mL at 36 h in a 5-L fermenter. Furthermore, the extracellular keratinase activity attained up to 16,860 U/mL via fed-batch fermentation within 30 h. The combination of keratinase with l-cysteine brings about 66.4 % degree of degradation of feather. Our work provides a new insight into the development of efficient keratinase fermentation processes with B. subtilis cell factory.