Enhanced production of poly-γ-glutamic acid via optimizing the expression cassette of Vitreoscilla hemoglobin in Bacillus licheniformis

Genetically engineering central carbon and nitrogen metabolism in Bacillus paralicheniformis for high γ-PGA production via glutamate-independent pathway

Poly-γ-glutamic acid (γ-PGA) is a biopolymer with great significance and broad applications. However, its fermentation in Bacillus strains usually requires costly glutamate supplementation. To reduce production costs, we engineered B. paralicheniformis to efficiently produce γ-PGA in a glutamate-independent manner by enhancing de novo glutamate biosynthesis via rocG-mediated pathways and alleviating carbon/nitrogen catabolite repression. In this study, we deleted three key regulatory genes: ccpA (catabolite control protein A), cggR (central glycolytic genes repressor), and tnrA (master regulator of nitrogen assimilation) to relieve repression on the TCA cycle, glycolysis, and γ-PGA biosynthesis pathways, thereby significantly increasing γ-PGA yield and productivity in B. paralicheniformis. In batch fermentation experiments, strains ΔccpA and ΔtnrA achieved γ-PGA yields of 63.86 g/L and 61.38 g/L, with productivities of 19.50 g/(L·h) and 6.93 g/(L·h) during the rapid increase period of γ-PGA production, respectively. To our knowledge, the ΔccpA strain achieved the highest γ-PGA yield and productivity among glutamate-independent producers. Both ΔccpA and ΔtnrA strains demonstrated superior efficiency in converting inexpensive NaNO₃ into high-value γ-PGA compared to the parental strain. Collectively, relieving suppression of carbon metabolism (the TCA cycle and glycolysis) and nitrogen metabolism (nitrate reduction) via genetic engineering holds significant potential for further enhancing the ability to biosynthesize γ-PGA in a glutamate-independent manner. The findings highlight the effectiveness of targeted genetic modifications in improving industrial bioprocesses for cost-efficient γ-PGA production.

Transcriptomics-guided rational engineering in Bacillus licheniformis for enhancing poly-γ-glutamic acid biosynthesis using untreated molasses

The utilization of non-food raw materials for microbial synthesis of poly-γ-glutamic acid (γ-PGA) presents a promising alternative to conventional food-based biosynthesis. However, the complex carbon source and composition of molasses, a prevalent non-food raw material, may impose constraints on its conversion and utilization by microorganisms. This study aimed to enhance the capacity of Bacillus licheniformis to convert untreated molasses into γ-PGA through transcriptomic analysis to guide metabolic modifications. Initial results from the transcriptomic analysis indicated that the strain utilizing molasses exhibited decreased expression of genes associated with substrate utilization (Module 1) and by-product synthesis (Module 2), while upregulating genes related to precursor synthesis (Module 3). Furthermore, we performed a knockout of the acetolactate synthase (AlsS) to reduce the synthesis of metabolic by-products and a knockout of the global regulator (CcpA) to alleviate carbon catabolite repression (CCR) and then promote substrate utilization. Ultimately, following the tandem overexpression of precursor supplying key genes, the titer of γ-PGA reached 48.26 g/L with a productivity of 1.15 g/L/h, using untreated molasses as the sole carbon source, which was 3.12-fold of the starting strain. These findings offer significant insights into the cost-effective synthesis of γ-PGA by bioconversion of untreated molasses during fermentation.

Understanding energy fluctuation during the transition state: The role of AbrB in Bacillus licheniformis

BACKGROUND

Limited research has been conducted on energy fluctuation during the transition state, despite the critical role of energy supply in microbial physiological metabolism.

RESULTS

This study aimed to investigate the regulatory function of transition state transcription factor AbrB on energy metabolism in Bacillus licheniformis WX-02. Firstly, the deletion of abrB was found to prolong the cell generation time, significantly reducing the intercellular ATP concentration and NADH/NAD+ ratio at the early stage. Subsequently, various target genes and transcription factors regulated by AbrB were identified through in vitro verification assays. Specifically, AbrB was shown to modulate energy metabolism by directly regulating the expression of genes pyk and pgk in substrate-level phosphorylation, as well as genes narK and narGHIJ associated with nitrate respiration. In terms of oxidative phosphorylation, AbrB not only directly regulated ATP generation genes, including cyd, atpB, hmp, ndh, qoxA and sdhC, but also influenced the expression of NAD-dependent enzymes and intracellular NADH/NAD+ ratio. Additionally, AbrB positively affected the expression of transcription factors CcpN, Fnr, Rex, and ResD involved in energy supply, while negatively affected the regulator CcpA. Overall, this study found that AbrB positively regulates both substrate-level phosphorylation and oxidative phosphorylation, while negatively regulating nitrate respiration.

CONCLUSIONS

This study proposes a comprehensive regulatory network of AbrB on energy metabolism in Bacillus, expanding the understanding of regulatory mechanisms of AbrB and elucidating energy fluctuations during the transition state.

Molecular weight control of poly-γ-glutamic acid reveals novel insights into extracellular polymeric substance synthesis in Bacillus licheniformis

BACKGROUND

The structural diversity of extracellular polymeric substances produced by microorganisms is attracting particular attention. Poly-gamma-glutamic acid (γ-PGA) is a widely studied extracellular polymeric substance from Bacillus species. The function of γ-PGA varies with its molecular weight (Mw).

RESULTS

Herein, different endogenous promoters in Bacillus licheniformis were selected to regulate the expression levels of pgdS, resulting in the formation of γ-PGA with Mw values ranging from 1.61 × 103 to 2.03 × 104 kDa. The yields of γ-PGA and exopolysaccharides (EPS) both increased in the pgdS engineered strain with the lowest Mw and viscosity, in which the EPS content was almost tenfold higher than that of the wild-type strain. Subsequently, the compositions of EPS from the pgdS engineered strain also changed. Metabolomics and RT-qPCR further revealed that improving the transportation efficiency of EPS and the regulation of carbon flow of monosaccharide synthesis could affect the EPS yield.

CONCLUSIONS

Here, we present a novel insight that increased pgdS expression led to the degradation of γ-PGA Mw and changes in EPS composition, thereby stimulating EPS and γ-PGA production. The results indicated a close relationship between γ-PGA and EPS in B. licheniformis and provided an effective strategy for the controlled synthesis of extracellular polymeric substances.

Engineering functional homopolymeric amino acids: from biosynthesis to design

Homopolymeric amino acids (HPAs) are a class of microbial polymers that can be classified into two categories: anionic and cationic HPAs. Notable examples include γ-poly-glutamic acid (γ-PGA) and ε-poly-L-lysine (ε-PL) that have wide-ranging applications in medicine, food, and agriculture. The primary method of manufacture is through microbial synthesis. In recent decades significant efforts have been made to enhance the production of HPAs, specifically focusing on γ-PGA and ε-PL. We comprehensively review current advances in understanding the synthetic mechanisms as well as metabolic engineering and fermentation process techniques to improve the production of HPAs. In addition, we discuss the major challenges and solutions associated with desired structure regulation of HPAs and the development of novel structures.

Enhanced poly-γ-glutamic acid synthesis in Corynebacterium glutamicum by reconstituting PgsBCA complex and fermentation optimization

Previously, a novel Corynebacterium glutamicum strain for the de novo biosynthesis of tailored poly-γ-glutamic acid (γ-PGA) has been constructed by our group. The strain was based on the γ-PGA synthetase complex, PgsBCA, which is the only polyprotein complex responsible for γ-PGA synthesis in Bacillus spp. In the present study, PgsBCA was reconstituted and overexpressed in C. glutamicum to further enhance γ-PGA synthesis. First, we confirmed that all the components (PgsB, PgsC, and PgsA) of γ-PGA synthetase derived from B. licheniformis are necessary for γ-PGA synthesis, and γ-PGA was detected only when PgsB, PgsC, and PgsA were expressed in combination in C. glutamicum. Next, the expression level of each pgsB, pgsC, and pgsA was tuned in order to explore the effect of expression of each of the γ-PGA synthetase subunits on γ-PGA production. Results showed that increasing the transcription levels of pgsB or pgsC and maintaining a medium-level transcription level of pgsA led to 35.44% and 76.53% increase in γ-PGA yield (γ-PGA yield-to-biomass), respectively. Notably, the expression level of pgsC had the greatest influence (accounting for 68.24%) on γ-PGA synthesis, followed by pgsB. Next, genes encoding for PgsC from four different sources (Bacillus subtilis, Bacillus anthracis, Bacillus methylotrophicus, and Bacillus amyloliquefaciens) were tested in order to identify the influence of PgsC-encoding orthologues on γ-PGA production, but results showed that in all cases the synthesis of γ-PGA was significantly inhibited. Similarly, we also explored the influence of gene orthologues encoding for PgsB on γ-PGA production, and found that the titer increased to 17.14 ± 0.62 g/L from 8.24 ± 0.10 g/L when PgsB derived from B. methylotrophicus replaced PgsB alone in PgsBCA from B. licheniformis. The resulting strain was chosen for further optimization, and we achieved a γ-PGA titer of 38.26 g/L in a 5 L fermentor by optimizing dissolved oxygen level. Subsequently, by supplementing glucose, γ-PGA titer increased to 50.2 g/L at 48 h. To the best of our knowledge, this study achieved the highest titer for de novo production of γ-PGA from glucose, without addition of L-glutamic acid, resulting in a novel strategy for enhancing γ-PGA production.

Promoting cell growth for bio-chemicals production via boosting the synthesis of L/D-alanine and D-alanyl-D-alanine in Bacillus licheniformis

Metabolic engineering is a substantial approach for escalating the production of biochemical products. Cell biomass is lowered by system constraints and toxication carried on by the aggregation of metabolites that serve as inhibitors of product synthesis. In order to increase the production of biochemical products, it is important to trace the relationship between alanine metabolism and biomass. According to our investigation, the appropriate concentration of additional L/D-alanine (0.1 g/L) raised the cell biomass (OD₆₀₀) in Bacillus licheniformis in contrast to the control strain. Remarkably, it was also determined that high levels of intracellular L/D-alanine and D-alanyl-D-alanine were induced by the overexpression of the ald, dal, and ddl genes to accelerate cell proliferation. Our findings clearly revealed that 0.2 g/L of L-alanine and D-alanine substantially elevated the titer of poly-γ-glutamic acid (γ-PGA) by 14.89% and 6.19%, correspondingly. And the levels of γ-PGA titer were hastened by the overexpression of the ald, dal, and ddl genes by 19.72%, 15.91%, and 16.64%, respectively. Furthermore, overexpression of ald, dal, and ddl genes decreased the by-products (acetoin, 2,3-butanediol, acetic acid and lactic acid) formation by about 14.10%, 8.77%, and 8.84% for augmenting the γ-PGA production. Our results also demonstrated that overexpression of ald gene amplified the production of lichenysin, pulcherrimin and nattokinase by about 18.71%, 19.82% and 21.49%, respectively. This work delineated the importance of the L/D-alanine and D-alanyl-D-alanine synthesis to the cell growth and the high production of bio-products, and provided an effective strategy for producing bio-products.