Optimal Secretory Expression of Acetaldehyde Dehydrogenase from Issatchenkia terricola in Bacillus subtilis through a Combined Strategy

Genome analysis of a newly isolated Bacillus velezensis-YW01 for biodegrading acetaldehyde

Acetaldehyde (AL), a primary carcinogen, not only pollutes the environment, but also endangers human health after drinking alcohol. Here a promising bacterial strain was successfully isolated from a white wine cellar pool in the province of Shandong, China, and identified as Bacillus velezensis-YW01 with 16 S rDNA sequence. Using AL as sole carbon source, initial AL of 1 g/L could be completely biodegraded by YW01 within 84 h and the cell-free extracts of YW01 has also been detected to biodegrade the AL, which indicate that YW01 is a high-potential strain for the biodegradation of AL. The optimal culture conditions and the biodegradation of AL of YW01 are at pH 7.0 and 38 °C, respectively. To further analyze the biodegradation mechanism of AL, the whole genome of YW01 was sequenced. Genes ORF1040, ORF1814 and ORF0127 were revealed in KEGG, which encode for acetaldehyde dehydrogenase. Furthermore, ORF0881 and ORF052 encode for ethanol dehydrogenase. This work provides valuable information for exploring metabolic pathway of converting ethanol to AL and subsequently converting AL to carboxylic acid compounds, which opened up potential pathways for the development of microbial catalyst against AL.

Extracellular Overexpression of a Neutral Pullulanase in Bacillus subtilis through Multiple Copy Genome Integration and Atypical Secretion Pathway Enhancement

Neutral pullulanases, having a good application prospect in trehalose production, showed a limited expression level. In order to address this issue, two approaches were utilized to enhance the yield of a new neutral pullulanase variant (PulA3E) in B. subtilis. One involved using multiple copies of genome integration to increase its expression level and fermentation stability. The other focused on enhancing the PulA-type atypical secretion pathway to further improve the secretory expression of PulA3E. Several strains with different numbers of genome integrations, ranging from one to four copies, were constructed. The four-copy genome integration strain PD showed the highest extracellular pullulanase activity. Additionally, the integration sites ytxE, ytrF, and trpP were selected based on their ability to enhance the PulA-type atypical secretion pathway. Furthermore, overexpressing the predicated regulatory genes comEA and yvbW of the PulA-type atypical secretion pathway in PD further improved its extracellular expression. Three-liter fermenter scale-up production of PD and PD-ARY yielded extracellular pullulanase activity of 1767.1 U/mL at 54 h and 2465.1 U/mL at 78 h, respectively. Finally, supplementing PulA3E with 40 U/g maltodextrin in the multi-enzyme catalyzed system resulted in the highest trehalose production of 166 g/L and the substrate conversion rate of 83%, indicating its potential for industrial application.

Bacillus subtilis: current and future modification strategies as a protein secreting factory

Bacillus subtilis is regarded as a promising microbial expression system in bioengineering due to its high stress resistance, nontoxic, low codon preference and grow fast. The strain has a relatively efficient expression system, as it has at least three protein secretion pathways and abundant molecular chaperones, which guarantee its expression ability and compatibility. Currently, many proteins are expressed in Bacillus subtilis, and their application prospects are broad. Although Bacillus subtilis has great advantages compared with other prokaryotes related to protein expression and secretion, it still faces deficiencies, such as low wild-type expression, low product activity, and easy gene loss, which limit its large-scale application. Over the years, many researchers have achieved abundant results in the modification of Bacillus subtilis expression systems, especially the optimization of promoters, expression vectors, signal peptides, transport pathways and molecular chaperones. An optimal vector with a suitable promoter strength and other regulatory elements could increase protein synthesis and secretion, increasing industrial profits. This review highlights the research status of optimization strategies related to the expression system of Bacillus subtilis. Moreover, research progress on its application as a food-grade expression system is also presented, along with some future modification and application directions.

Enhanced extracellular production of raw starch-degrading α-amylase in Bacillus subtilis through expression regulatory element modification and fermentation optimization

BACKGROUND

Raw starch-degrading α-amylase (RSDA) can hydrolyze raw starch at moderate temperatures, thus contributing to savings in starch processing costs. However, the low production level of RSDA limits its industrial application. Therefore, improving the extracellular expression of RSDA in Bacillus subtilis, a commonly used industrial expression host, has great value.

RESULTS

In this study, the extracellular production level of Pontibacillus sp. ZY raw starch-degrading α-amylase (AmyZ1) in B. subtilis was enhanced by expression regulatory element modification and fermentation optimization. As an important regulatory element of gene expression, the promoter, signal peptide, and ribosome binding site (RBS) sequences upstream of the amyZ1 gene were sequentially optimized. Initially, based on five single promoters, the dual-promoter P_veg-P_ylB was constructed by tandem promoter engineering. Afterward, the optimal signal peptide SP_NucB was obtained by screening 173 B. subtilis signal peptides. Then, the RBS sequence was optimized using the RBS Calculator to obtain the optimal RBS1. The resulting recombinant strain WBZ-VY-B-R1 showed an extracellular AmyZ1 activity of 4824.2 and 41251.3 U/mL during shake-flask cultivation and 3-L fermenter fermentation, which were 2.6- and 2.5-fold greater than those of the original strain WBZ-Y, respectively. Finally, the extracellular AmyZ1 activity of WBZ-VY-B-R1 was increased to 5733.5 U/mL in shake flask by optimizing the type and concentration of carbon source, nitrogen source, and metal ions in the fermentation medium. On this basis, its extracellular AmyZ1 activity was increased to 49082.1 U/mL in 3-L fermenter by optimizing the basic medium components as well as the ratio of carbon and nitrogen sources in the feed solution. This is the highest production level reported to date for recombinant RSDA production.

CONCLUSIONS

This study represents a report on the extracellular production of AmyZ1 using B. subtilis as a host strain, and achieved the current highest expression level. The results of this study will lay a foundation for the industrial application of RSDA. In addition, the strategies employed here also provide a promising way for improving other protein production in B. subtilis.

A genetic toolkit for efficient production of secretory protein in Bacillus subtilis

Bacillus subtilis is a microbial cell factory widely used to produce recombinant proteins, but the expression of heterologous proteins is often severely hampered. This study constructed a genetic toolkit for improving the secretory efficiency of heterologous proteins in Bacillus subtilis. First, the protease-deficient hosts were reconstructed. Then, two endogenous constitutive promoters, P_hag and P_spovG, were screened. Next, a method called systemic combinatorial optimization of ribosome binding site (RBS) equipped with signal peptide (SCORES) was designed for optimizing the secretion and translation of the heterologous protein. Finally, Serratia marcescens nonspecific endonuclease (SMNE), which causes cell death by degrading nucleic acids, was expressed. The enzyme activity in the shake flask reached 7.5 × 10⁶ U/L, which was 7.5-times that of the control RBS and signal peptide combination (RS0). This study not only expanded on the synthetic biology toolbox in B. subtilis but also provided strategies to create a prokaryotic protein expression system.