Optimal secretion of thermostable Beta-glucosidase in Bacillus subtilis by signal peptide optimization

Enhanced lignocellulose degradation in Bacillus subtilis RLI2019 through CRISPR/Cas9-mediated chromosomal integration of ternary cellulase genes

Bacillus subtilis (B. subtilis) is a crucial industrial microorganism for lignocellulose biomass degradation. However, wild-type strains from natural environments have inherent deficiencies in the composition of cellulase genes, so constructing recombinant strains through genome engineering is a generalizable strategy to overcome these shortcomings. Herein, eglS, cel48S, and bglS were integrated into the aprE, epr, and amyE loci of the B. subtilis RLI2019 chromosome, respectively, through CRISPR/Cas9-mediated genome editing, deriving the engineered strain B. subtilis AEA3. The activities of endoglucanase, exoglucanase, β-glucosidase, xylanase, and total cellulase in B. subtilis AEA3 were enhanced by 3.1-fold, 6.6-fold, 3.0-fold, 1.2-fold, and 1.8-fold, respectively, reaching 26.31 U/mL, 9.77 U/mL, 3.91 U/mL, 19.63 U/mL, and 2.42 U/mL. Notably, the engineered strain improved the saccharification efficiency of crop straws, effectively disrupting fiber structure, and significantly reducing the content of neutral and acid detergent fibers, lignocellulose and hemicellulose. In summary, this study provides a general strategy for enhancing the cellulose degradation capabilities of B. subtilis through comprehensive and systematic multi-module genetic engineering, broadening its potential application in lignocellulose biomass conversion.

Recombinant extracellular expression of β-glucosidase in Bacillus subtilis and its potential secretion mechanism

β-Glucosidase is an important specific β-1,4 glycosidic bond hydrolase. Here, β-glucosidase Bgl2A achieved extracellular secretory expression in Bacillus subtilis mediated by the signal peptide SPAmyQ, and its extracellular activity was increased 3.35-fold by signal peptide optimization. Interestingly, the recombinant extracellular activity of Bgl2A without signal peptide mediation was 5.05-fold higher than that with SPAmyQ mediation, and its stability was better at acidic conditions and low temperatures. Combining the results of classical secretory pathway knockout and cell integrity assays, Bgl2A without signal peptide can perform non-classical extracellular secretion in B. subtilis. Notably, the hydrophobic region I266-Q269 is a critical sequence affecting the non-classical secretion of Bgl2A. Mutants I267A and I267E increased the extracellular activity and secretion rate of Bgl2A by 1.42- and 1.58-fold, respectively. In addition, maintaining the hydrophobicity of the amino acids at positions 268 and 269 may be essential for the non-classical secretion of Bgl2A. To our knowledge, this study first reported that SPLipB was not detected at the N-terminus of the intracellular Bgl2A precursor from WBLBBgl-SPLipB mediated by the Sec-type signal peptide SPLipB. This offers a theoretical foundation for enhancing recombinant extracellular expression of β-glucosidase in B. subtilis and understanding the mechanism of non-classical secretion in this organism.

High-efficiency heterologous expression of functional RVG29-BoNT/A light chain fusion protein in Bacillus subtilis based on a combinatorial strategy

Botulinum neurotoxin type A (BoNT/A) has been used as an injectable therapeutic agent in medical aesthetics, neuroscience research, and other medical fields. In this study, the non-endotoxin-producing and food-safe bacterium Bacillus subtilis WB800N was selected as the host strain to express functional RVG29-BoNT/A light chain (Lc) fusion proteins through a combinatorial strategy involving promoter engineering, signal peptide optimization, and overexpression of transport-related proteins and molecular chaperones. The RVG29 peptide, known for its neuronal targeting ability, was fused to the Lc of BoNT/A to enhance its transmembrane delivery and substrate cleavage activity. Furthermore, a series of promoter screening, tandem promoter construction, and site-specific mutations were conducted to maximize transcriptional activity of the target protein. Moreover, a library of 244 native signal peptides was constructed, and high-throughput fluorescence screening identified Sec pathway-related signal peptides that significantly improved secretion efficiency. Meanwhile, transport-related proteins, particularly FtsY, and molecular chaperones like GroEL/DnaJ, were overexpressed to further enhance the secretion and soluble expression of the RVG29-Lc. Additionally, knockdown of the hrcA gene, a repressor of heat-shock genes, improved the expression of molecular chaperones, as well as the RVG29-Lc. The combinatorial strategy achieved a final intracellular expression level of 500.33 ± 5.00 mg/L and the secretion level of 75.15 ± 4.33 mg/L under the same experimental conditions. This work presents a robust framework for the high-efficiency production of functional RVG29-Lc fusion proteins in B. subtilis, offering valuable insights into the industrial-scale production of therapeutic BoNT-derived proteins.

Functional small peptides for enhanced protein delivery, solubility, and secretion in microbial biotechnology

In microbial biotechnology, there is a constant demand for functional peptides to give new functionality to engineered proteins to address problems such as direct delivery of functional proteins into bacterial cells, enhanced protein solubility during the expression of recombinant proteins, and efficient protein secretion from bacteria. To tackle these critical issues, we selected three types of functional small peptides: cell-penetrating peptides (CPPs) enable the delivery of diverse cargoes into bacterial cytoplasm for a variety of purposes, protein-solubilizing peptide tags demonstrate remarkable efficiency in solubilizing recombinant proteins without folding interference, and signal peptides play a key role in enabling the secretion of recombinant proteins from bacterial cells. In this review, we introduced these three functional small peptides that offer effective solutions to address emerging problems in microbial biotechnology. Additionally, we summarized various engineering efforts aimed at enhancing the activity and performance of these peptides, thereby providing valuable insights into their potential for further applications.

Enhancing the secretion of a feruloyl esterase in Bacillus subtilis by signal peptide screening and rational design

Feruloyl esterase is a subclass of α/β hydrolase, which could release ferulic acid from biomass residues for use as an efficient additive in food or pharmaceutical industries. In the present study, a feruloyl esterase with broad substrate specificity was characterised and secreted by Bacillus subtilis WB600. After codon usage optimisation and signal peptide library screening, the secretion amount of feruloyl esterase was enhanced by up to 10.2-fold in comparison with the base strain. The site-specific amino acid substitutions that facilitate protein folding further improved the secretion by about 1.5-fold. The purified rationally designed enzyme exhibited maximal activity against methyl ferulate at pH 6.5 and 65 °C. In the solid-state fermentation, the genetically engineered B. subtilis released about 37% of the total alkali-extractable ferulic acid in maize bran. This study provides a promising candidate for ferulic acid production and demonstrates that the secretion of a heterologous enzyme from B. subtilis can be cumulatively improved by changes in protein sequence features.

Enhanced Extracellular Production and Characterization of Sucrose Isomerase in Bacillus subtilis with Optimized Signal Peptides

Sucrose isomerase (SIase) catalyzes the hydrolysis and isomerization of sucrose into isomaltulose, which is an important functional sugar widely used in the food industry. However, the lack of safe and efficient expression systems for recombinant SIase has impeded its production and application. In this study, enhanced expression of a SIase from Klebsiella sp. LX3 (referred to as KsLX3-SIase) was achieved in Bacillus subtilis WB800N, by optimizing the signal peptides. First, 13 candidate signal peptides were selected using a semi-rational approach, and their effects on KsLX3-SIase secretion were compared. The signal peptide WapA was most efficient in directing the secretion of KsLX3-SIase into the culture medium, producing a specific activity of 23.0 U/mL, as demonstrated by shake flask culture. Using a fed-batch strategy, the activity of KsLX3-SIase in the culture medium was increased to 125.0 U/mL in a 5-L fermentor. Finally, the expressed KsLX3-SIase was purified and was found to have maximum activity at 45 °C and pH 5.5. Its K_m for sucrose was 267.6 ± 18.6 mmol/L, and its k_cat/K_m was 10.1 ± 0.2 s⁻¹mM⁻¹. These findings demonstrated an efficient expression of SIase in B. subtilis, and this is thought to be the highest level of SIase produced in a food-grade bacteria to date.