Novel Cell Wall Hydrolase CwlC from Bacillus thuringiensis Is Essential for Mother Cell Lysis

Identification and investigation of the effects of N-acetylmuramoyl-L-alanine amidase in Bacillus amyloliquefaciens for the cell lysis and heterologous protein production

Bacillus amyloliquefaciens (BA) is considered as an important industrial strain for heterologous proteins production. However, its severe autolytic behavior leads to reduce the industrial production capacity of the chassis cells. In this study, we aimed to evaluate the autolysis of N-acetylmuranyl-L-alanine amidase in BA TCCC11018, and further slowed down the cell lysis for improved the heterologous protein production by a series of modifications. Firstly, we identified six N-acetylmuramic acid-L-alanines by bioinformatics, and analyzed the transcriptional levels at different culture time points by transcriptome and quantitative real-time PCR. Then, by establishing an efficient CRISPR-nCas9 gene editing method, N-acetylmuramic acid-L-alanine genes were knocked out or overexpressed to verify its effect on cell lysis. Then, by single or tandem knockout N-acetylmuramic acid-L-alanines, it was determined that the reasonable modification of LytH and CwlC1 can slow down cell lysis. After 48 h of culture, the autolysis rate of the mutant strain BA ΔlytH-cwlC1 decreased by 4.83 %, and the amylase activity reached 176 U/mL, which was 76.04 % higher than that of the control strain BA Δupp. The results provide a reference for mining the functional characteristics of autolysin in Bacillus spp., and provide from this study reveal valuable insights delaying the cell lysis and increasing heterologous proteins production.

Characterization of a novel cell wall hydrolase CwlE involved in Bacillus thuringiensis subsp. israelensis mother cell lysis

Cell wall hydrolases are ubiquitous among spore-form bacteria and essential for mother cell lysis. In this study, a novel cell wall hydrolase gene cwlE involved in mother cell lysis was characterized from Bacillus thuringiensis subsp. israelensis (Bti) strain Bt-59. cwlE was specifically expressed in Bti and located in the large plasmid carrying the insecticidal genes. The encoded CwlE protein consists of a MurNAc-LAA domain and two highly conserved catalytic residues (E26 and E151). The recombinant CwlE-His protein was able to digest the cell wall of Bti, indicating that CwlE is an N-acetylmuramoyl-L-alanine amidase. Transcriptional analysis indicated that cwlE began to express at the early stage of stationary phase and was controlled by SigE. Single mutation of cwlE gene delayed Bti mother cell lysis, while double mutation of cwlE and sigK completely blocked Bti mother cell lysis. After exposure to UV light to deactivate the crystal proteins, the level of decrease of insecticidal activity against mosquito larvae of Bt-59 (ΔcwlE-sigK) was less than that observed for Bt-59. This study elucidates the mechanism of Bti mother cell lysis and provides an effective strategy for mosquito control using Bt products with increased persistence.

Key amino acids residues enhance the ability of CpcR to activate cry gene expression in Bacillus thuringiensis

Typical Bacillus thuringiensis (Bt) produces one or more parasporal crystals composed of insecticidal Cry proteins during the sporulation, and the parasporal crystals and spores are produced from the same cell. Strain Bt LM1212 is different from typical Bt strains in that its crystals and spores are produced in different cells. Previous studies have found that the cell differentiation process of Bt LM1212 is related to the transcription factor CpcR which activates the cry-gene promoters. In addition, CpcR could activate the Bt LM1212 cry35-like gene promoter (P35) when introduced in the heterologous HD73- strain. It was shown that P35 was only activated in non-sporulating cells. In this study, the peptidic sequences of CpcR homologous proteins found in other strains of the Bacillus cereus group were used as references to identify two key amino acid sites for CpcR activity. The function of these amino acids was investigated by measuring P35 activation by CpcR in strain HD73-. These results will lay a foundation for the optimization of the insecticidal protein expression system in non-sporulating cells.

Utilization of a strong promoter combined with the knockout of protease genes to improve the yield of Vip3Aa in Bacillus thuringiensis BMB171

BACKGROUND

Vip3Aa is an insecticidal protein secreted by some Bacillus thuringiensis strains during vegetative growth. It has excellent insecticidal activity, its mechanism of action is different from that of Cry protein, and it can delay the development of pest resistance. To date, Vip3Aa has been widely used in genetically modified Bt crops. However, the secretion of Vip3Aa by industrial production strains is usually very low. Moreover, most of the Vip3Aa in the medium is degraded by proteases, limiting its application as a biopesticide.

RESULTS

We report a novel constitutive strong promoter from B. thuringiensis, P_rsi , which directs the abundant expression of vip3Aa in B. thuringiensis BMB171. Furthermore, to reduce the degradation of Vip3Aa caused by proteases, we constructed B. thuringiensis mutants in which different protease genes were knocked out. We found that the degradation of Vip3Aa was greatly inhibited and its yield was significantly improved in a mutant that lacked all three protease genes.

CONCLUSION

Our results provide a new strategy to enhance the production of Vip3Aa in B. thuringiensis and have reference value for the research and development of novel bioinsecticides. © 2023 Society of Chemical Industry.

Sporulation, Structure Assembly, and Germination in the Soil Bacterium Bacillus thuringiensis: Survival and Success in the Environment and the Insect Host

Bacillus thuringiensis (Bt) is a rod-shaped, Gram-positive soil bacterium that belongs to the phylum Firmicutes and the genus Bacillus. It is a spore-forming bacterium. During sporulation, it produces a wide range of crystalline proteins that are toxic to different orders of insects. Sporulation, structure assembly, and germination are essential stages in the cell cycle of B. thuringiensis. The majority of studies on these issues have focused on the model organism Bacillus subtilis, followed by Bacillus cereus and Bacillus anthracis. The machinery for sporulation and germination extrapolated to B. thuringiensis. However, in the light of recent findings concerning the role of the sporulation proteins (SPoVS), the germination receptors (Gr), and the cortical enzymes in Bt, the theory strengthened that conservation in sporulation, structure assembly, and germination programs drive the survival and success of B. thuringiensis in the environment and the insect host. In the present minireview, the latter pinpointed and reviewed.

Methylarsenite is a broad-spectrum antibiotic disrupting cell wall biosynthesis and cell membrane potential

Methylarsenite (MAs(III)), a product of arsenic biomethylation or bioreduction of methylarsenate (MAs(V)), has been proposed as a primitive antibiotic. However, the antibacterial property and the bactericidal mechanism of MAs(III) remain largely unclear. In this study, we found that MAs(III) is highly toxic to 14 strains of bacteria, especially against 9 strains of Gram-positive bacteria with half maximal inhibitory concentration (IC50) in the sub micromolar range for Staphyloccocus aureus, Microbacterium sp., Pseudarthrobacter siccitolerans and several Bacillus species. In a co-culture of B. subtilis 168 and MAs(III)-producer Enterobacter sp. CZ-1, the later reduced non-toxic MAs(V) to highly toxic MAs(III) to kill the former and gain a competitive advantage. MAs(III) induced autolysis of B. subtilis 168. Deletion of the autolysins LytC, LytD, LytE, and LytF suppressed MAs(III)-induced autolysis in B. subtilis 168. Transcriptomic analysis showed that MAs(III) downregulated the expression of the major genes involved in the biosynthesis of the cell wall peptidoglycan. Overexpression of an UDP-N-acetylglucosamine enolpyruvyl transferase gene murAA alleviated MAs(III)-induced autolysis in B. subtilis 168. MAs(III) disrupted the membrane potential of B. subtilis 168 and caused severe membrane damage. The results suggest that MAs(III) is a broad-spectrum antibiotic preferentially against Gram-positive bacteria by disrupting the cell wall biosynthesis pathway and cell membrane potential.

Deletion of the novel gene mother cell lysis X results in Cry1Ac encapsulation in the Bacillus thuringiensis HD73

The novel protein MclX (mother cell lysis X) in Bacillus thuringiensis subsp. kurstaki strain HD73 (B. thuringiensis HD73) was characterized in this work. MclX has no known domain and its gene deletion in HD73 resulted in Cry1Ac encapsulation in the mother cell and did not influence Cry1Ac protein production or insecticidal activity. In vitro cell wall hydrolysis experiments showed that MclX cannot hydrolyze the cell wall. In mclX deletion mutants, the expression of cwlC (which encodes a key cell wall hydrolase) was significantly decreased, as shown by the β-galactosidase activity assay. MclX cannot directly bind to the cwlC promoter, based on the electrophoretic mobility shift assay (EMSA). The cwlC was reported to be regulated by σK and GerE. However, the transcriptional activities of sigK and gerE showed no difference between HD73 and the mclX deletion mutant. It is indicated that MclX influenced cwlC expression independently of σK or GerE, through a new pathway to regulate cwlC expression. mclX deletion could be a new approach for insecticidal protein encapsulation in Bacillus thuringiensis.

Constitutive chitosanase from Bacillus thuringiensis B-387 and its potential for preparation of antimicrobial chitooligomers

The article proves the ability of the entomopathogenic strain B. thuringiensis var. dendrolimus B-387 to high the constitutive production (3-12.5 U/mL) of extracellular chitosanase, that was found for the first time. The enzyme was purified in 94-fold by ultrafiltration, affinity sorption and cation-exchange chromatography and characterized biochemically. The molecular mass of the chitosanase determined using SDS-PAGE is 40 kDa. Temperature and pH-optima of the enzyme are 55 ^°C and pH 6.5, respectively; the chitosanase was stable under 50-60 ^°C and pH 4-10.5. Purified chitosanase most rapidly (V_max ~ 43 µM/mL × min, K_M ~ 0.22 mg/mL, k_cat ~ 4.79 × 10⁴ s^-1) hydrolyzed soluble chitosan of the deacetylation degree (DD) 85% by endo-mode, and did not degrade colloidal chitin, CM-cellulose and some other glucans. The main reaction products of the chitosan enzymolysis included chitobiose, chitotriose and chitotetraose. In addition to small chitooligosaccharides (CHOs), the studied chitosanase also generated low-molecular weight chitosan (LMWC) with average M_w in range 14-46 kDa and recovery 14-35%, depending on the enzyme/substrate ratio and incubation temperature. In some cases, the chitosan (DD 85 and 50%) oligomers prepared using crude chitosanase from B. thuringiensis B-387 indicated higher antifungal and antibacterial activities in vitro in comparison with the initial polysaccharides. The data obtained indicate the good prospect of chitosanase B-387 for the production of bioactive CHOs.

ARTP and NTG compound mutations improved Cry protein production and virulence of Bacillus thuringiensis X023

A high production mutated strain Bacillus thuringiensis X023PN (BtX023PN) was screened from the wild strain Bacillus thuringiensis X023 (BtX023) after atmospheric and room temperature plasma (ARTP) and nitrosoguanidine (NTG) mutation. BtX023PN grows faster than the wild strain, and its lysis of mother cell was 6 h ahead BtX023, but the ability of sporulation was significantly reduced. Bioassay indicated that compared with the wild type strain, the virulence of BtX023PN against Plutella xylostella (P. xylostella) and Mythimna seperata (M. seperata) increased to 2.33-fold and 2.13-fold respectively. qRT-PCR and SDS-PAGE demonstrated that the production of Cry1Ac increased by 61%. Resequence indicated that the mutated sites enriched on the key carbohydrate metabolism and amino acid metabolism. This study provides a new strain resource for the development of Bt insecticides and a feasible technical strategy for the breeding of Bt. KEY POINTS: • Atmospheric and room temperature plasma used in breeding of Bacillus thuringiensis. • Less stationary phase time with more ICP production. • Semi-lethal concentration against Plutella xylostella reduced by about 57.

Identification and Functional Characterization of Two Homologous SpoVS Proteins Involved in Sporulation of Bacillus thuringiensis

Sporulation is an important part of the life cycle of Bacillus thuringiensis and the basis for the production of parasporal crystals. This study identifies and characterizes two homologous spoVS genes (spoVS1 and spoVS2) in B. thuringiensis, both of whose expression is dependent on the σ^H factor. The disruption of spoVS1 and spoVS2 resulted in defective B. thuringiensis sporulation. Similar to Bacillus subtilis, B. thuringiensis strain HD(ΔspoVS1) mutants showed delayed formation of the polar septa, decreased sporulation efficiency, and blocked spore release. Different from B. subtilis, B. thuringiensis HD(ΔspoVS1) mutants had disporic septa and failed to complete engulfment in some cells. Moreover, HD(ΔspoVS2) mutants had delayed spore release. The effect of spoVS1 deletion on polar septum delay and sporulation efficiency could be compensated by spoVS2. β-Galactosidase activity analysis showed that the expression of pro-sigE and spoIIE decreased to different degrees in the HD(ΔspoVS1) and HD(ΔspoVS2) mutants. The different effects of the two mutations on the expression of sporulation genes led to decreases in Cry1Ac production of different levels.

IMPORTANCE There is only one spoVS gene in B. subtilis, and its effects on sporulation have been reported. In this study, two homologous spoVS genes were found and identified in B. thuringiensis. The different effects on sporulation and parasporal crystal protein production in B. thuringiensis and their relationship were investigated. We found that these two homologous spoVS genes are highly conserved in the Bacillus cereus group, and therefore, the functional characterization of SpoVS is helpful to better understand the sporulation processes of members of the Bacillus cereus group.

Characterization of CwlC, an autolysin, and its role in mother cell lysis of Bacillus thuringiensis subsp. israelensis

Bacillus thuringiensis subsp. israelensis (Bti) has been proven to efficiently control mosquitoes, of which many species are important vectors of human disease. The larvicidal action is attributed to the parasporal crystals formed in the sporulating cells and released upon cell autolysis. In this study, a sporulation-specific cwlC gene that encodes an N-acetylmuramoyl-_L -alanine amidase was characterized in Bti strain Bt-59. CwlC was the only cell wall hydrolase in Bti found to contain both MurNAc-LAA and Amidase02_C domains. A recombinant CwlC-His protein was able to digest the Bacillus cell wall. Deletion of the cwlC gene delayed Bti mother cell lysis without impacting vegetative growth or insecticidal efficacy. Transcriptional analyses indicated that cwlC was expressed at the late sporulation stage and was controlled by SigK. Two other cell wall hydrolase genes, cwlB and cwlE, with high expression levels at T₁₄ in Bt-59, were also identified. Like cwlC, cwlB expression was controlled by SigK; in contrast, cwlE was found not to be under the control of this sigma factor and unlike the other two, its gene was found to be plasmid encoded.

Phenotypic and genotypic characterization of the new Bacillus cereus phage SWEP1

A new Bacillus cereus phage, SWEP1, was isolated from black soil. The host lysis activity of phage SWEP1 has a relatively short latent time (20 min) and a small burst size of 83 PFU. The genome of SWEP1 consists of 162,461 bp with 37.77% G+C content. The phage encodes 278 predicted proteins, 103 of which were assigned functionally. No tRNA genes were found. Comparative genomics analysis indicated that SWEP1 is related to Bacillus phage B4 (86.91% identity, 90% query coverage). Phenotypic and genotypic characterization suggested that SWEP1 is a new member of a new species in the genus Bequatrovirus, family Herelleviridae.

Transition Phase Regulator AbrB Positively Regulates the sip1Ab1 Gene Expression in Bacillus thuringiensis

Bacillus thuringiensis secreted insecticidal proteins (Sip) are a secretion that is toxic to coleopteran pests. However, the transcriptional mechanism of sip genes is still unknown. The transcriptional regulation of the sip1Ab1 gene and the expression of the Sip1Ab1 protein were investigated in this study. The results demonstrated that the secretion of the Sip1Ab1 protein in HD73 was almost the same as that in the original QZL38 strain during the transition phase. Analysis of the β-galactosidase activities of sip1Ab1-lacZ in both the HD73 and abrB mutant strains indicated that the transcription of sip1Ab1 is dependent on AbrB. Electrophoretic mobility shift assays showed that AbrB could bind with the sip1Ab1 promoter, and two binding sites of AbrB in the region of the promoter of sip1Ab1 were determined by DNase I footprinting assays. All of the above-described results proved that AbrB positively regulates the sip1Ab1 gene.

IMPORTANCEBacillus thuringiensis Sip proteins are secreted insecticidal toxins that are toxic to coleopteran pests. In this study, we investigated the transcriptional mechanism of the sip gene and showed strong evidence that Sip1Ab1 is secreted in the transition phase and that AbrB, a transition phase regulator that is usually a repressor, positively and directly regulates sip1Ab1. Reports of AbrB positive regulation are rare, even in Bacillus subtilis. To the best of our knowledge, no toxic gene has been reported to be positively regulated by AbrB in Bacillus species.

Evolution, purification, and characterization of RC0497: a peptidoglycan amidase from the prototypical spotted fever species Rickettsia conorii

Rickettsial species have independently lost several genes owing to reductive evolution while retaining those predominantly implicated in virulence, survival, and biosynthetic pathways. In this study, we have identified a previously uncharacterized Rickettsia conorii gene RC0497 as an N-acetylmuramoyl-L-alanine amidase constitutively expressed during infection of cultured human microvascular endothelial cells at the levels of both mRNA transcript and encoded protein. A homology-based search of rickettsial genomes reveals that RC0497 homologs, containing amidase_2 family and peptidoglycan binding domains, are highly conserved among the spotted fever group (SFG) rickettsiae. The recombinant RC0497 protein exhibits α-helix secondary structure, undergoes a conformational change in the presence of zinc, and exists as a dimer at higher concentrations. We have further ascertained the enzymatic activity of RC0497 via demonstration of its ability to hydrolyze Escherichia coli peptidoglycan. Confocal microscopy on E. coli expressing RC0497 and transmission immunoelectron microscopy of R. conorii revealed its localization predominantly to the cell wall, septal regions of replicating bacteria, and the membrane of vesicles pinching off the cell wall. In summary, we have identified and functionally characterized RC0497 as a peptidoglycan hydrolase unique to spotted fever rickettsiae, which may potentially serve as a novel moonlighting protein capable of performing multiple functions during host-pathogen interactions.

Construction of a Conditionally Asporogenous Bacillus thuringiensis Recombinant Strain Overproducing Cry Protein by Deletion of the leuB Gene

One of the common shortcomings with Bacillus thuringiensis (Bt) biopesticides in field application is their instability under UV irradiation. In Bt, the leuB gene encodes the 3-isopropylmalate dehydrogenase. In addition to its role in leucine biosynthesis, LeuB would be likely recruited to catalyze the dehydrogenation of malate in the final step of tricarboxylic acid cycle during sporulation. In this study, we constructed a Bt recombinant strain in which the gene leuB was deleted by using the markerless gene deletion system. The ΔleuB mutant strain showed a conditionally asporogenous phenotype while overproducing insecticidal crystal proteins and retaining its insecticidal activity well in both fermentation and LB media. Furthermore, the metabolic regulation mechanisms of LeuB was elucidated by iTRAQ-based quantitative proteomics approach. Evidences from proteomics data suggested that the inhibited supply of pyruvate (carbon source) was an important factor related to the conditionally asporogenous feature of the mutant. Consistently, the mutant regained its ability to sporulate in LB medium by adding 1% glucose or 1% sodium pyruvate. Taken together, our study demonstrated that deletion of the leuB gene resulted in delayed or completely blocked mother cell lysis, allowing the crystals encapsulated within cells, which makes this recombinant strain a good candidate for developing Bt preparations with better UV-stability.

Characteristics of the sigK Deletion Mutant from Bacillus thuringiensis var. israelensis Strain Bt-59

All major insecticidal genes of Bacillus thuringiensis var. israelensis (Bti) are controlled by the sporulation-specific sigma factor Sigma E (sigE), while sigE is negatively regulated by Sigma K (sigK). Therefore, knocking out sigK plays an important role in regulating the expression of insecticidal genes in Bti. A sigK deletion mutant of B. thuringiensis var. israelensis strain Bt-59, Bt59(ΔsigK), was constructed by homologous recombination and characterized. The sigK deletion resulted in no mature spores and delayed mother cell lysis from T₂₅ to T₆₀, while the genetically complemented strain, Bt59(HFsigK), had mother cell lysis at T₂₅. Compared to Bt-59, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis indicated that the expression of Cry4Aa2/4Ba1 and Cyt1Aa1 proteins in Bt59(ΔsigK) increased approximately 1.67 and 1.21 times, respectively. However, there was no significant change in Cry11Aa1 protein expression between the two strains. Bioassay results showed that the sigK deletion mutation slightly reduced the insecticidal activity of Bt-59 against Culex pipiens pallens and did not obviously affect activity against Aedes albopictus.

2-Methylcitrate cycle: a well-regulated controller of Bacillus sporulation

Bacillus thuringiensis is the most widely used eco-friendly biopesticide, containing two primary determinants of biocontrol, endospore and insecticidal crystal proteins (ICPs). The 2-methylcitrate cycle is a widespread carbon metabolic pathway playing a crucial role in channelling propionyl-CoA, but with poorly understood metabolic regulatory mechanisms. Here, we dissect the transcriptional regulation of the 2-methylcitrate cycle operon prpCDB and report its unprecedented role in controlling the sporulation process of B. thuringiensis. We found that the transcriptional activity of the prp operon encoding the three critical enzymes PrpC, PrpD, and PrpB in the 2-methylcitrate cycle was negatively regulated by the two global transcription factors CcpA and AbrB, while positively regulated by the LysR family regulator CcpC, which jointly account for the fact that the 2-methylcitrate cycle is specifically and highly active in the stationary phase of growth. We also found that the prpD mutant accumulated 2-methylcitrate, the intermediate metabolite of the 2-methylcitrate cycle, which delayed and inhibited sporulation at the early stage. Thus, our results not only revealed sophisticated transcriptional regulatory mechanisms for the metabolic 2-methylcitrate cycle but also identified 2-methylcitrate as a novel regulator of sporulation in B. thuringiensis.

Effect of the spoIIID mutation on mother cell lysis in Bacillus thuringiensis

SpoIIID is a small, sequence-specific DNA-binding protein which can direct many genes' transcription and has an effect on spore formation in Bacillus subtilis. We investigated the role of SpoIIID in mother cell lysis in Bacillus thuringiensis. A β-galactosidase assay based on the promoter fusions with lacZ indicated that the sigK gene was positively regulated by SpoIIID and σ^K negatively regulated the expression of sigE. The spoIIID mutant strain exhibited no mother cell lysis in Schaeffer's sporulation medium (SSM) but did in ½ Luria-Bertani (LB) medium. cwlC is an essential hydrolase gene for mother cell lysis. Moreover, the expression of a P_cwlC-lacZ fusion in spoIIID mutant was proved to be higher in ½ LB medium than in SSM. HD (ΔspoIIID)(ΔcwlC) mutant was obtained by knocking out the cwlC gene in HD(ΔspoIIID) and displayed no mother cell lysis in both SSM and ½ LB mediums. The deletion of spoIIID decreased the crystal protein production in HD73. The expression of P_orf1cry8E and P₅₀₁₄ promoter fusions with lacZ gene in the acrystalliferous HD^-(ΔspoIIID) mutant showed similar activity to that in the acrystalliferous HD73^- strain before T₇ and slightly higher than that in the acrystalliferous HD73^- after T₇. Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that Cry1Ac production in HD^-(ΔspoIIID) directed by the P_orf1cry8E and P₅₀₁₄ promoters was at a similar level as that in HD73 wild strain. Altogether, these results suggested that the spoIIID mutant with P_orf1cry8E or P₅₀₁₄ promoters could be an alternative delivery system for cry gene expression with no mature spore formation and medium-dependent mother cell lysis.