Deletion of aprA and nprA genes for alkaline protease A and neutral protease A from bacillus thuringiensis: effect on insecticidal crystal proteins

Addition of proteinase K during the culture alter the physiology of Bacillus thuringiensis culture and the cry1Ac, nprX, nprA, and spo0A gene transcription

Bacillus thuringiensis is the major bioinsecticide worldwide produced due to the Cry protein activity. Several studies have been done to improve the cost-productivity relation. The neutral protease A (NprA) is the major extracellular protein massively produced during the stationary phase by this bacterium, contributing to the Cry proteins' degradation. Also, the deletion of aprA and nprA genes enhanced the yield of Cry protein, stabilizing it. Therefore, to increase Cry production, one possibility is to degrade the NprA protease in the culture media. In the present study, proteinase K was used to hydrolyze the NprA to increase Cry production. Proteinase K was added during the exponential growth of B. thuringiensis culture. The bacilli and endospores were measured along all culture, while the Cry protein was measured at the end of the culture. The addition of PK affects the bacilli and spore kinetics positively but negatively to the Cry protein (there is no Cry protein detection). Therefore, the gene expression of the cry1Ac, nprX, nprA, and spo0A was measured. The expression of each gene was followed along all culture. Results demonstrated that PK alters both the transcriptional levels and the expression order of the genes.

Effect of lon Protease Overexpression on Endotoxin Production and Stress Resistance in Bacillus thuringiensis

Lon protease, an intracellular protease, plays a key role in cell homeostasis in bacteria and is involved in numerous physiological processes. In this work, we aimed to study the impact of Lon on the production of endotoxins and stress response in Bacillus thuringiensis, which is an important bioinsecticide alternative for toxic chemicals. For this purpose, lon gene was cloned into a multi-copy vector with its original promoter and transcriptional terminator and expressed in B. thuringiensis serovar israelensis ATCC 35,646. Our results showed that the recombinant lon gene transcribed and translated efficiently and the resulting protein was active. Although the sporulation efficiency of the recombinant strain was found to be reduced and its mobility impaired, overexpression of the lon gene triggered the production of endotoxin. Together with increased biofilm formation, recombinant strain exhibited significantly better adaptation to osmotic and heat shock stresses and UV exposure compared to wild type and the control strain with empty plasmid. This study suggested a possible link between Lon protease and the production of insecticide and stress response in B. thuringiensis and provides a platform for future studies focusing on enhancing bio-insecticidal production using this bacterium.

Modulation of Cas9 level for efficient CRISPR-Cas9-mediated chromosomal and plasmid gene deletion in Bacillus thuringiensis

OBJECTIVES

To set up an efficient gene editing system in Bacillus thuringiensis (Bt) using CRISPR-Cas9 by demonstrating deletion of chromosomal and plasmid genes.

RESULTS

CRISPR-Cas9 from Streptococcus pyogenes was found to function in Bt cells, resulting in DNA cleavage that is lethal to the cells. The system was assessed for its ability to mediate gene editing by knock-out of the protease genes nprA (neutral protease A) and aprA (alkaline protease A). Gene editing was not detected when the Bacillus-derived pBCX was used to carry CRISPR-Cas9 elements and a DNA repair template. When the Cas9 promoter was replaced with the sporulation-specific promoter cyt2A, a Bt ∆nprA clone was obtained, but this plasmid construct did not give reproducible results. Bt ∆nprA ∆aprA and Bt ∆aprA deletion mutants were finally generated when the Lactobacillus plantarum-derived plasmid pLPPR9 was used, likely due to its lower copy number reducing Cas9 toxicity. Only three to four clones each needed to be screened to identify the desired gene-modified mutants. Conversely, efficient editing of the plasmid vip3A gene required the use of pBCX and longer homology sequences for the repair template.

CONCLUSIONS

Capitalizing on the differential impact of plasmid copy number and homology arm length, we devised distinct yet simple and efficient approaches to chromosomal and plasmid gene deletion for Bt that condense the screening process, minimize screening, and facilitate multiple consecutive gene editing steps.

Continuous evolution of Bacillus thuringiensis toxins overcomes insect resistance

The Bacillus thuringiensis δ-endotoxins (Bt toxins) are widely used insecticidal proteins in engineered crops that provide agricultural, economic, and environmental benefits. The development of insect resistance to Bt toxins endangers their long-term effectiveness. Here we have developed a phage-assisted continuous evolution selection that rapidly evolves high-affinity protein-protein interactions, and applied this system to evolve variants of the Bt toxin Cry1Ac that bind a cadherin-like receptor from the insect pest Trichoplusia ni (TnCAD) that is not natively bound by wild-type Cry1Ac. The resulting evolved Cry1Ac variants bind TnCAD with high affinity (dissociation constant Kd = 11-41 nM), kill TnCAD-expressing insect cells that are not susceptible to wild-type Cry1Ac, and kill Cry1Ac-resistant T. ni insects up to 335-fold more potently than wild-type Cry1Ac. Our findings establish that the evolution of Bt toxins with novel insect cell receptor affinity can overcome insect Bt toxin resistance and confer lethality approaching that of the wild-type Bt toxin against non-resistant insects.

An improved, high-quality draft genome sequence of the Germination-Arrest Factor-producing Pseudomonas fluorescens WH6

Background

Pseudomonas fluorescens is a genetically and physiologically diverse species of bacteria present in many habitats and in association with plants. This species of bacteria produces a large array of secondary metabolites with potential as natural products. P. fluorescens isolate WH6 produces Germination-Arrest Factor (GAF), a predicted small peptide or amino acid analog with herbicidal activity that specifically inhibits germination of seeds of graminaceous species.

Results

We used a hybrid next-generation sequencing approach to develop a high-quality draft genome sequence for P. fluorescens WH6. We employed automated, manual, and experimental methods to further improve the draft genome sequence. From this assembly of 6.27 megabases, we predicted 5876 genes, of which 3115 were core to P. fluorescens and 1567 were unique to WH6. Comparative genomic studies of WH6 revealed high similarity in synteny and orthology of genes with P. fluorescens SBW25. A phylogenomic study also placed WH6 in the same lineage as SBW25. In a previous non-saturating mutagenesis screen we identified two genes necessary for GAF activity in WH6. Mapping of their flanking sequences revealed genes that encode a candidate anti-sigma factor and an aminotransferase. Finally, we discovered several candidate virulence and host-association mechanisms, one of which appears to be a complete type III secretion system.

Conclusions

The improved high-quality draft genome sequence of WH6 contributes towards resolving the P. fluorescens species, providing additional impetus for establishing two separate lineages in P. fluorescens. Despite the high levels of orthology and synteny to SBW25, WH6 still had a substantial number of unique genes and represents another source for the discovery of genes with implications in affecting plant growth and health. Two genes are demonstrably necessary for GAF and further characterization of their proteins is important for developing natural products as control measure against grassy weeds. Finally, WH6 is the first isolate of P. fluorescens reported to encode a complete T3SS. This gives us the opportunity to explore the role of what has traditionally been thought of as a virulence mechanism for non-pathogenic interactions with plants.

Activity of spores and extracellular proteins from six Cry+strains and a Cry–strain ofBacillus thuringiensissubsp.kurstakiagainst the western spruce budworm,Choristoneura occidentalis(Lepidoptera: Tortricidae)

We characterized insecticidal activity of previously untested strains of Bacillus thuringiensis kurstaki belonging to two crystal serovars (K-1 and K-73) against the western spruce budworm ( Choristoneura occidentalis Freeman 1967). By testing various components, we demonstrated that spores play a critical role in the pathogenesis of each strain. Spore-free crystals caused low mortality and purified spores were generally not toxic. The addition of spores to purified protoxin increased toxicity several hundred-fold, regardless of the parental strain from which the spores or protoxins were derived. The crystal and spore components did not account for full insecticidal activity of whole sporulated cultures owing to the toxicity of soluble proteins that are secreted during cell growth. We observed a marked difference in toxicity of secreted proteins between the K-1 and K-73 type strains, with the K-1 preparations causing much higher mortality, mass reduction, and inhibition of pupation. There was a consistent correlation between relative toxicity of secreted protein preparations and the presence and quantity of the Vip3A protein, suggesting that this protein contributes to the virulence of B. thuringiensis subsp. kurstaki in western spruce budworm larvae. However, other virulence factors have to be invoked to explain the synergizing effect of spores from both K-1 and K-73 strains on Cry protein toxicity.

Bioconversion of industrial wastewater and wastewater sludge into Bacillus thuringiensis based biopesticides in pilot fermentor

Starch industry wastewater (SWW), slaughterhouse wastewater (SHWW) and secondary sludges from three different wastewater treatment plants (Jonquière--JQS, Communauté Urbaine de Québec--CUQS and Black lake-BLS) were used as raw materials for the production of Bacillus thuringiensis (Bt) based biopesticides in a pilot scale fermentor (100 L working volume). The slaughterhouse wastewater exhibited the lowest Bt growth and entomotoxcity (Tx) potential (measured against spruce budworm) due to low availability of carbon, nitrogen and other nutrients. Performance variation (growth, sporulation, proteolytic activity and Tx potential) within the three types of sludges was directly related to the availability of nitrogen and carbohydrates, which could change with sludge origin and methods employed for its generation. The Tx potential of Bt obtained in different secondary sludges (JQS: 12 x 10(9) SBU/L; CUQS: 13 x 10(9) SBU/L and BLS: 16 x 10(9) SBU/L) and SWW (18 x 10(9) SBU/L) was higher than the soybean based synthetic medium (10 x 10(9) SBU/L). The maximum protease activity was obtained in CUQ secondary sludge (4.1 IU/mL) due to its high complex protein concentration. Nevertheless, high carbohydrate concentration in SWW repressed enzyme production. The secondary sludges and SWW were found to be suitable raw materials for high potency Bt biopesticide production.

Discovery and characterization of Sip1A: a novel secreted protein from Bacillus thuringiensis with activity against coleopteran larvae

Bioassay screening of Bacillus thuringiensis culture supernatants identified strain EG2158 as having larvicidal activity against Colorado potato beetle (Leptinotarsa decemlineata) larvae. Ion-exchange fractionation of the EG2158 culture supernatant resulted in the identification of a protein designated Sip1A (secreted insecticidal protein) of approximately 38 kDa having activity against Colorado potato beetle (CPB). An oligonucleotide probe based on the N-terminal sequence of the purified Sip1A protein was used to isolate the sip1A gene. The sequence of the Sip1A protein, as deduced from the sequence of the cloned sip1A gene, contained 367 residues (41,492 Da). Recombinant B. thuringiensis and Escherichia coli harboring cloned sip1A produced Sip1A protein which had insecticidal activity against larvae of CPB, southern corn rootworm (Diabrotica undecimpunctata howardi), and western corn rootworm (Diabrotica virgifera virgifera).

Effects of the 20-kilodalton helper protein on Cry1Ac production and spore formation in Bacillus thuringiensis

Bacillus thuringiensis produces large amounts of various pesticidal proteins during the stationary phase. In order to achieve a high yield and form crystals, some pesticidal proteins require the presence of other proteins. Helper protein P20 is required for efficient production of both the Cyt1A and Cry11A crystal proteins in B. thuringiensis subsp. israelensis. Although full-length Cry1 protoxins are usually independent in terms of expression and crystallization in B. thuringiensis, in this study P20 significantly enhanced production of Cry1Ac protoxin (133 kDa) in an acrystalliferous and plasmid-negative strain. In the presence of P20, the yield of Cry1Ac protoxin increased 2.5-fold, and on average the resulting crystals were 1.85 microm long and 0.85 microm wide, three times the size of the crystals formed in the control lacking P20. Correspondingly, the recombinant strain that coexpressed P20 and Cry1Ac exhibited higher toxicity against Heliothis armigera larvae than the control. Furthermore, serious degradation of Cry1Ac in vivo was observed, which has seldom been reported previously. Actually, most protein was completely degraded during synthesis, and after synthesis about one-third of the expressed protoxins were degraded further before crystallization. In this process, P20 protected only nascent Cry1Ac from degradation, indicating that it acted as a molecular chaperon. In addition, spores were smaller and rounder and had a thinner exosporium layer when they were produced in the presence of P20. In summary, Cry1Ac was severely degraded during synthesis; this degradation was effectively relieved by P20, which resulted in enhanced production. Our results indicated that P20 is an effective tool for optimizing protein production in vivo.

Gene knockout demonstrates that vip3A contributes to the pathogenesis of Bacillus thuringiensis toward Agrotis ipsilon and Spodoptera exigua

Vip3A is an 89-kDa protein secreted by Bacillus thuringiensis during vegetative growth. To determine the importance of Vip3A for the insect pathogenicity of B. thuringiensis the vip3A gene was deleted from strain HD1, yielding strain HD1Deltavip3A. Compared with HD1, strain HD1Deltavip3A was one-fourth as toxic to Agrotis ipsilon larvae and less than one-tenth as toxic to Spodoptera exigua larvae. When streptomycin was included in the S. exigua diet the toxicity of HD1Deltavip3A was approximately half that of HD1. Addition of HD1 spores increased the toxicity of purified Cry1 protein more than 600-fold against S. exigua, whereas addition of HD1Deltavip3A spores increased toxicity of Cry1 protein approximately 10-fold. These results demonstrate that an important component of B. thuringiensis insecticidal activity against S. exigua is the synthesis of Vip3A protein by B. thuringiensis cells after ingestion of spores and crystal proteins by insect larvae.