Bacillus Amyloliquefaciens LL3 Research Articles

Engineering of a genome-reduced strain Bacillus amyloliquefaciens for enhancing surfactin production

BackgroundGenome reduction and metabolic engineering have emerged as intensive research hotspots for constructing the promising functional chassis and various microbial cell factories. Surfactin, a lipopeptide-type biosurfactant with broad spectrum antibiotic activity, has wide application prospects in anticancer therapy, biocontrol and bioremediation. Bacillus amyloliquefaciens LL3, previously isolated by our lab, contains an intact srfA operon in the genome for surfactin biosynthesis.ResultsIn this study, a genome-reduced strain GR167 lacking ~ 4.18% of the B. amyloliquefaciens LL3 genome was constructed by deleting some unnecessary genomic regions. Compared with the strain NK-1 (LL3 derivative, ΔuppΔpMC1), GR167 exhibited faster growth rate, higher transformation efficiency, increased intracellular reducing power level and higher heterologous protein expression capacity. Furthermore, the chassis strain GR167 was engineered for enhanced surfactin production. Firstly, the iturin and fengycin biosynthetic gene clusters were deleted from GR167 to generate GR167ID. Subsequently, two promoters PRsuc and PRtpxi from LL3 were obtained by RNA-seq and promoter strength characterization, and then they were individually substituted for the native srfA promoter in GR167ID to generate GR167IDS and GR167IDT. The best mutant GR167IDS showed a 678-fold improvement in the transcriptional level of the srfA operon relative to GR167ID, and it produced 311.35 mg/L surfactin, with a 10.4-fold increase relative to GR167.ConclusionsThe genome-reduced strain GR167 was advantageous over the parental strain in several industrially relevant physiological traits assessed and it was highlighted as a chassis strain for further genetic modification. In future studies, further reduction of the LL3 genome can be expected to create high-performance chassis for synthetic biology applications.

Metabolic engineering of Bacillus amyloliquefaciens LL3 for enhanced poly-γ-glutamic acid synthesis.

SummaryPoly‐γ‐glutamic acid (γ‐PGA) is a biocompatible and biodegradable polypeptide with wide‐ranging applications in foods, cosmetics, medicine, agriculture and wastewater treatment. Bacillus amyloliquefaciens LL3 can produce γ‐PGA from sucrose that can be obtained easily from sugarcane and sugar beet. In our previous work, it was found that low intracellular glutamate concentration was the limiting factor for γ‐PGA production by LL3. In this study, the γ‐PGA synthesis by strain LL3 was enhanced by chromosomally engineering its glutamate metabolism‐relevant networks. First, the downstream metabolic pathways were partly blocked by deleting fadR, lysC, aspB, pckA, proAB, rocG and gudB. The resulting strain NK‐A6 synthesized 4.84 g l−1 γ‐PGA, with a 31.5% increase compared with strain LL3. Second, a strong promoter PC 2up was inserted into the upstream of icd gene, to generate strain NK‐A7, which further led to a 33.5% improvement in the γ‐PGA titre, achieving 6.46 g l−1. The NADPH level was improved by regulating the expression of pgi and gndA. Third, metabolic evolution was carried out to generate strain NK‐A9E, which showed a comparable γ‐PGA titre with strain NK‐A7. Finally, the srf and itu operons were deleted respectively, from the original strains NK‐A7 and NK‐A9E. The resulting strain NK‐A11 exhibited the highest γ‐PGA titre (7.53 g l−1), with a 2.05‐fold improvement compared with LL3. The results demonstrated that the approaches described here efficiently enhanced γ‐PGA production in B. amyloliquefaciens fermentation.

Enhanced production of antifungal lipopeptide iturin A by Bacillus amyloliquefaciens LL3 through metabolic engineering and culture conditions optimization

BackgroundIturins, which belong to antibiotic cyclic lipopeptides mainly produced by Bacillus sp., have the potential for application in biomedicine and biocontrol because of their hemolytic and antifungal properties. Bacillus amyloliquefaciens LL3, isolated previously by our lab, possesses a complete iturin A biosynthetic pathway as shown by genomic analysis. Nevertheless, iturin A could not be synthesized by strain LL3, possibly resulting from low transcription level of the itu operon.ResultsIn this work, enhanced transcription of the iturin A biosynthetic genes was implemented by inserting a strong constitutive promoter C2up into upstream of the itu operon, leading to the production of iturin A with a titer of 37.35 mg l−1. Liquid chromatography-mass spectrometry analyses demonstrated that the strain produced four iturin A homologs with molecular ion peaks at m/z 1044, 1058, 1072 and 1086 corresponding to [C14 + 2H]2+, [C15 + 2H]2+, [C16 + 2H]2+ and [C17 + 2H]2+. The iturin A extract exhibited strong inhibitory activity against several common plant pathogens. The yield of iturin A was improved to 99.73 mg l−1 by the optimization of the fermentation conditions using a response surface methodology. Furthermore, the yield of iturin A was increased to 113.1 mg l−1 by overexpression of a pleiotropic regulator DegQ.ConclusionsTo our knowledge, this is the first report on simultaneous production of four iturin A homologs (C14–C17) by a Bacillus strain. In addition, this study suggests that metabolic engineering in combination with culture conditions optimization may be a feasible method for enhanced production of bacterial secondary metabolites.

Effects of MreB paralogs on poly-γ-glutamic acid synthesis and cell morphology in Bacillus amyloliquefaciens.

Actin-like MreB paralogs play important roles in cell shape maintenance, cell wall synthesis and the regulation of the D,L-endopeptidases, CwlO and LytE. The gram-positive bacteria, Bacillus amyloliquefaciens LL3, is a poly-γ-glutamic acid (γ-PGA) producing strain that contains three MreB paralogs: MreB, Mbl and MreBH. In B. amyloliquefaciens, CwlO and LytE can degrade γ-PGA. In this study, we aimed to test the hypothesis that modulating transcript levels of MreB paralogs would alter the synthesis and degradation of γ-PGA. The results showed that overexpression or inhibition of MreB, Mbl or MreBH had distinct effects on cell morphology and the molecular weight of the γ-PGA products. In fermentation medium, cells of mreB inhibition mutant were 50.2% longer than LL3, and the γ-PGA titer increased by 55.7%. However, changing the expression level of mbl showed only slight effects on the morphology, γ-PGA molecular weight and titer. In the mreBH inhibition mutant, γ-PGA production and its molecular weight increased by 56.7% and 19.4%, respectively. These results confirmed our hypothesis that suppressing the expression of MreB paralogs might reduce γ-PGA degradation, and that improving the cell size could strengthen γ-PGA synthesis. This is the first report of enhanced γ-PGA production via suppression of actin-like MreB paralogs.

A markerless gene replacement method for B. amyloliquefaciens LL3 and its use in genome reduction and improvement of poly-γ-glutamic acid production.

We herein adapted a markerless gene replacement method by combining a temperature-sensitive plasmid pKSV7 with a counterselectable marker, the upp gene encoding uracil phosphoribosyltransferase (UPRTase), for the poly-γ-glutamic acid (γ-PGA)-producing strain Bacillus amyloliquefaciens LL3. Deletion of the upp gene conferred LL3 5-fluorouracil (5-FU) resistance. Sensitivity to 5-FU was restored when LL3 Δupp was transformed with pKSV7-based deletion plasmid which carries a functional allele of the upp gene of Bacillus subtilis 168. These observations allowed us to adapt a two-step plasmid integration and excision strategy to perform markerless deletion of genes of interest. Deletion plasmid harboring a mutant allele of the target gene was first integrated in the genome by culturing cells under nonpermissive conditions for pKSV7 replication. Single-crossover recombinants were then grown without antibiotics to aid the second recombinational event. 5-FU was used to select for double-crossover recombinants with plasmid evicted from the chromosome. The resulting recombinants either harbored the wild-type or mutated allele of the target gene and could be identified by PCR and DNA sequencing. Using this method, we successively removed the amyA gene and a 47-kb fragment of the bae cluster from the genome of LL3, with higher efficiency compared with previous reports. We also investigated the effects of a transcriptional regulator, RocR, on γ-PGA production and cell growth. Specific γ-PGA production of the rocR mutant was increased by 1.9-fold, which represents a new way to improve γ-PGA production.

Functions of poly-gamma-glutamic acid (γ-PGA) degradation genes in γ-PGA synthesis and cell morphology maintenance.

Poly-γ-glutamic acid (γ-PGA) is an important biopolymer with greatly potential in industrial and medical applications. In the present study, we constructed a metabolically engineered glutamate-independent Bacillus amyloliquefaciens LL3 strain with considerable γ-PGA production, which was carried out by single, double, and triple markerless deletions of three degradation genes pgdS, ggt, and cwlO. The highest γ-PGA production (7.12 g/L) was obtained from the pgdS and cwlO double-deletion strain NK-pc, which was 93 % higher than that of wild-type LL3 strain (3.69 g/L). The triple-gene-deletion strain NK-pgc showed a 28 % decrease in γ-PGA production, leading to a yield of 2.69 g/L. Furthermore, the cell morphologies of the mutant strains were also characterized. The cell length of cwlO deletion strains NK-c and NK-pc was shorter than that of the wild-type strain, while the ggt deletion strains NK-g, NK-pg, NK-gc, and NK-pgc showed longer cell lengths. This is the first report concerning the markerless deletion of γ-PGA degradation genes to improve γ-PGA production in a glutamate-independent strain and the first observation that γ-glutamyltranspeptidase (encoded by ggt) could be involved in the inhibition of cell elongation.

Curing the Plasmid pMC1 from the Poly (γ-glutamic Acid) Producing Bacillus amyloliquefaciens LL3 Strain Using Plasmid Incompatibility

Bacillus amyloliquefaciens LL3 is a glutamate-independent poly-γ-glutamic acid (γ-PGA) producing strain which consists of a circular chromosome (3,995,227bp) and an endogenous plasmid pMC1 (6,758bp). The study of the function of native plasmid and the genome-size reduction of the B. amyloliquefaciens LL3 strain requires elimination of the endogenous plasmid. Traditional plasmid-curing procedures using sodium dodecyl sulfate (SDS) or acridine orange combined with heat treatment have been shown to be ineffective in this strain. Plasmid incompatibility is an effective method for curing which has been studied before. In our research, the hypothetical Rep protein gene and the origin of replication of the endogenous plasmid were cloned into the temperature-sensitive vector yielding the incompatible plasmid pKSV7-rep-ori. This plasmid was transformed into LL3 by electroporation. The analysis of the strain bearing incompatible plasmids after incubation at 30°C for 30 generations showed the production of plasmid cured strains. High frequency of elimination was achieved with more than 93% of detected strains showing to be plasmid-cured. This is the first report describing plasmid cured in a γ-PGA producing strain using this method. The plasmid-cured strains showed an increase of γ-PGA production by 6% and led to a yield of 4.159g/l, compared to 3.918g/l in control and cell growth increased during the early stages of the exponential phase. Gel permeation chromatography (GPC) characterization revealed that the γ-PGA produced by plasmid-cured strains and the wild strains were identical in terms of molecular weight. What is more, the further study of plasmid function showed that curing of the endogenous plasmid did not affect its sporulation efficiency.

Chromosome integration of theVitreoscillahemoglobin gene (vgb) mediated by temperature-sensitive plasmid enhances γ-PGA production inBacillus amyloliquefaciens

Oxygen is a limiting factor in the production of γ-PGA by the glutamic acid-independent strain Bacillus amyloliquefaciens LL3 because of the high viscosity of the culture broth. The vgb gene encoding Vitreoscilla hemoglobin (VHb) was introduced into LL3 to overcome the low concentration of dissolved oxygen (DO). First, recombinant plasmid pWHV was constructed by cloning vgb into the Bacillus expression vector pWH1520 and transformed into LL3. Carbon monoxide difference spectral analysis confirmed the expression of VHb. The γ-PGA yield of LL3 (pWHV) under the optimized fermentation conditions increased by 9.56%. To overcome the instability of pWH1520 and to establish stable expression of VHb, the engineered strain LL3-PVK was constructed by homologous recombination between the integration vector pKSVPVK and the 16S rRNA gene of LL3. The temperature-sensitive plasmid was used to perform the integration, which successfully circumvented the obstacle of the low transformation efficiency of B.amyloliquefaciens LL3. Bacillus amyloliquefaciens LL3-PVK showed an increase of 30% in γ-PGA production, while the biomass was increased by 7.9%. To our knowledge, this is the first report describing enhancement of γ-PGA production in a glutamic acid-independent strain as a result of vgb expression.

Glutamic acid independent production of poly-γ-glutamic acid by Bacillus amyloliquefaciens LL3 and cloning of pgsBCA genes

A new glutamic acid independent poly-γ-glutamic acid (γ-PGA) producing strain, which was identified as Bacillusamyloliquefaciens LL3 by analysis of 16S rDNA and gyrase subunit A gene (gyrA), was isolated from fermented food. The product had a molecular weight of 470, 801 and l-glutamate monomer content of 98.47%. The pre-optimal medium, based on single-factor tests and orthogonal design, contained 50g/L sucrose, 2g/L (NH4)2SO4, 0.6g/L MgSO4, and provided well-balanced changes in processing parameters and a γ-PGA yield of 4.36g/L in 200L system. The γ-PGA synthetase genes pgsBCA were cloned from LL3, and successfully expressed by pTrcLpgs vector in Escherichia coli JM109, resulting the synthesis of γ-PGA without glutamate. This study demonstrates the designedly improved yield of γ-PGA in 200L system and the first report of pgsBCA from glutamic acid independent strain, which will benefit the metabolized mechanism investigation and the wide-ranging application of γ-PGA.