Extracellular secretion in Bacillus subtilis of a cytoplasmic thermostable β-galactosidase from Geobacillus stearothermophilus

β-Galactosidase catalyzes the hydrolysis of β-galactosides into monosaccharides and is widely used in dairy processing. This study reports the extracellular secretion of a cytoplasmic thermostable β-galactosidase from Geobacillus stearothermophilus IAM11001 in Bacillus subtilis. This enzyme has potential applications in the dairy industry. It was not secreted in B. subtilis by mediation of 3 general secretory signal peptides, but was secreted extracellularly when it was fused to a twin-arginine signal peptide of B. subtilis phosphodiesterase. Defined and rich culture media were used for recombinant enzyme production, and the extracellular target enzymatic activity reached about 44% of the total enzymatic activity synthesized at 18 h of cultivation in Luria-Bertani medium. As a control of secretion, when the signal peptide coding sequence was absent from the N terminus of the target gene bgaB, the extracellular target enzymatic activity obtained under the same condition of cultivation accounted for less than 7% of the total enzymatic activity synthesized. Results also showed that coexpression of the B. subtilis proteins TatAd and TatCd was indispensable for the secretion of the target enzyme.