Modulation of Cry IV A toxin protein expression by glucose in Bacillus thuringiensis israelensis

Regulation of cry gene expression in Bacillus thuringiensis

Bacillus thuringiensis differs from the closely related Bacillus cereus group species by its ability to produce crystalline inclusions. The production of these crystals mainly results from the expression of the cry genes, from the stability of their transcripts and from the synthesis, accumulation and crystallization of large amounts of insecticidal Cry proteins. This process normally coincides with sporulation and is regulated by various factors operating at the transcriptional, post-transcriptional, metabolic and post-translational levels.

Improvement of Bacillus thuringiensis bioinsecticide production by sporeless and sporulating strains using response surface methodology

Statistical experimental designs, involving a Plackett-Burman design followed by a rotatable central composite design were used to optimize the culture medium constituents for Bacillus thuringiensis bioinsecticide production. This was carried out by using firstly an asporogenic strain and extrapolated to some sporeless and sporulating strains. Initial screening of production parameters was performed and the variables with statistically significant effects on delta-endotoxin production were identified: glucose, glycerol, yeast extract and MnSO(4). These variables were selected for further optimization by response surface methodology. The obtained results revealed that the optimum culture medium for delta-endotoxin production consists of 22.5 g/l of glucose, 4.8g/l of glycerol, 5.8 g/l of yeast extract and 0.008 g/l of MnSO(4). Under these conditions, delta-endotoxin production was 2,130 and 2,260 mg/l into 250 and 1,000 ml flask respectively, which represent more than 38% improvement in toxin production over the basal medium (1,636 mg/l). Such medium composition was shown to be suitable for overproducing delta-endotoxins by sporeless and sporulating strains.

Enhanced recombinant protein production in pyruvate kinase mutant of Bacillus subtilis

Previous work demonstrated that acetate production was substantially lower in pyruvate kinase (pyk) mutant of Bacillus subtilis. The significantly lower acetate production in the pyk mutant is hypothesized to have positive effect on recombinant protein production either by lifting the inhibitory effect of acetate accumulation in the medium or redirecting the metabolic fluxes beneficial to biomass/protein synthesis. In this study, the impact of the pyk mutation on recombinant protein production was investigated. Green fluorescent protein (GFP+) was selected as a model protein and constitutively expressed in both the wild-type strain and a pyk mutant. In batch cultures, the pyk mutant produced 3-fold higher levels of recombinant protein when grown on glucose as carbon source. Experimental measurements and theoretical analysis show that the higher protein yield of the mutant is not due to removal of an acetate-associated inhibition of expression or gene dosage or protein stability but a much lower acetate production in the mutant allows for a greater fraction of carbon intake to be directed to protein synthesis.

Identification of a catabolite-responsive element necessary for regulation of the cry4A gene of Bacillus thuringiensis subsp. israelensis

Bacillus thuringiensis subsp. israelensis produces a potent mosquitocidal protein, Cry4A. We have identified a 15-bp catabolite responsive element (cre), overlapping the -35 element of the cry4A promoter. Changing a guanine to adenine at position -49 in the promoter abolished glucose catabolite repression of cry4A and enhanced promoter activity two- to threefold. This cis regulatory element is essential for controlled toxin synthesis, vital to evolutionary success of B. thuringiensis subsp. israelensis.

Fructose and glucose mediates enterotoxin production and anaerobic metabolism of Bacillus cereus ATCC14579(T)

AIMS

To determine the effects of carbohydrates on Bacillus cereus ATCC14579(T) anaerobic metabolism and enterotoxin production in amino acids rich medium.

METHODS AND RESULTS

Bacillus cereus anaerobic growth on different carbohydrates (glucose, fructose, sucrose or glucose-fructose mixture) was examined in synthetic mMOD medium under continuous cultures (mu = 0.2 h(-1)). Fermentation end-products, flux partitioning at each key branch points of the mixed acid pathway and consumption or production of amino acids were determined. On both fructose and sucrose, ATP production was favoured via acetate production from acetyl-CoA. In addition, amino acids present in the growth medium showed significant variations with high consumption of serine and net production of glutamate and alanine on some or all sugars. Enterotoxins Hbl and Nhe production was high during growth on fructose (or mixtures involving a fructose moiety).

CONCLUSIONS

Fructose was identified as a key sugar influencing anaerobic metabolism and toxin production of B. cereus.

SIGNIFICANCE AND IMPACT OF THE STUDY

The physiological differences associated with the fermentation of the various carbohydrates clearly modify toxinogenesis indicating that the risk of foodborne pathogens is to some extent dependent upon the prevailing nutritional environment.

The production of Bacillus cereus enterotoxins is influenced by carbohydrate and growth rate

Enterotoxin production is a key factor in Bacillus cereus food poisoning. Herein, the effect of the growth rate (mu) on B. cereus toxin production when grown on sucrose was studied and the Hemolytic BL enterotoxin (HBL) and nonhemolytic enterotoxin (Nhe) production by B. cereus was compared according to carbohydrate at mu = 0.2 h(-1). The anaerobic growth was carried out on continuous cultures in synthetic medium supplemented with glucose, fructose, sucrose, or an equimolar mixture of glucose and fructose. Concerning the HBL and Nhe enterotoxin production: (1) the highest enterotoxin production has occurred at mu = 0.2 h(-1) when growing on sucrose; (2) HBL production was repressed when glucose was consumed and the presence of fructose (alone or in mixture) cancelled glucose catabolite repression; (3) the consumption of sucrose increased Nhe production, which was not affected by the catabolite repression. Furthermore, analysis of the fermentative metabolism showed that whatever the mu or the carbon source, B. cereus used the mixed acid fermentation to ferment the different carbohydrates. The enterotoxin productions by this strain at mu = 0.2 h(-1) are highly influenced by the carbohydrates that do not involve any fermentative metabolism changes.

Response of larval Chironomus tepperi (Diptera: Chironomidae) to individual Bacillus thuringiensis var. israelensis toxins and toxin mixtures

The biopesticide Bacillus thuringiensis israelensis (B.t.i.) is highly toxic to the larvae of Chironomus tepperi, an important pest of aerially sown rice in southern Australia. In this study, all of the known Cry genes and the Cyt1A gene from B.t.i. were expressed and tested for individual toxicity against fourth instar C. tepperi larvae. Possible synergism between toxins in two component mixtures involving all toxins except Cry10A was also evaluated. Of the Cry toxins, only Cry11A and Cry4B displayed substantial toxicity; however, both were 10- to 20-fold less toxic than the parental B.t.i. strain. The only detected synergy was between the mildly toxic Cry4A and Cyt1A toxins. In direct contrast to previous studies with mosquitoes, mixtures of Cry11A/Cry4B and Cry11A/Cyt1A were mildly antagonistic. The activity of Cry11A and Cry4B is sufficient to justify investigation as to whether their expression in transgenic rice plants could provide control of C. tepperi larvae.

Anaerobiosis and low specific growth rates enhance hemolysin BL production by Bacillus cereus F4430/73

Bacillus cereus F4430/73 produced the highest levels of hemolysin BL (HBL) when grown under anaerobiosis in MOD medium. Anaerobic cells grown in a chemostat at low specific growth rate (0.1-0.2 h(-1)) expressed up to sevenfold more HBL than did cells held at a faster growth rate. At 0.2 h(-1), the presence of 90 mM glucose resulted in inhibition of HBL production. Glucose was found to repress HBL induction at the mRNA level, indicating the potential involvement of catabolite repression in the regulation of HBL. Based on these data, it is suggested that growth rate could be an effector of catabolite regulation of HBL.

Nutritional and cultural parameters influencing antidipteran delta-endotoxin production

In this study, various nutritional and cultural parameters influencing diptera-specific delta-endotoxin synthesis by Bacillus thuringiensis subsp. israelensis HD500 were investigated. Of various inorganic nitrogen sources, the highest yields of Cry11Aa and Cry4Ba proteins were obtained on (NH(4))(2)HPO(4). Among carbon sources, inulin, dextrin, maltose, lactose, sucrose, whey and glycerol were all stimulatory, while glucose, starch and molasses were suppressive. High concentrations of inorganic phosphate (50 to 100 mM K(2)HPO(4)) were required for an effective synthesis of Cry4Ba. Mn was the most critical element for the biosynthesis of both toxins at 10(-6) M concentration. Mg and Ca favored production when provided at 8 x 10(-3) M and 5.5 x 10(-4) M concentrations, respectively, while Fe, Zn and Cu negatively influenced biosynthesis. Cry4-toxin synthesis was best at neutral pH and also when the organism was grown at 25 degrees C. Throughout the study, the extent of growth and sporulation of the producer organism was also monitored.

Loss of catabolite repression function of HPr, the phosphocarrier protein of the bacterial phosphotransferase system, affects expression of the cry4A toxin gene in Bacillus thuringiensis subsp. israelensis

HPr, the phosphocarrier protein of the bacterial phosphotransferase system, mediates catabolite repression of a number of operons in gram-positive bacteria. In order to participate in the regulatory process, HPr is activated by phosphorylation of a conserved serine-46 residue. To study the potential role of HPr in the regulation of Cry4A protoxin synthesis in Bacillus thuringiensis subsp. israelensis, we produced a catabolite repression-negative mutant by replacing the wild-type copy of the ptsH gene with a mutated copy in which the conserved serine residue of HPr was replaced with an alanine. HPr isolated from the mutant strain was not phosphorylated at Ser-45 by HPr kinase, but phosphorylation at His-14 was found to occur normally. The enzyme I and HPr kinase activities of the mutant were not affected. Analysis of the B. thuringiensis subsp. israelensis mutant harboring ptsH-S45A in the chromosome showed that cry4A expression was derepressed from the inhibitory effect of glucose. The mutant strain produced both cry4A and sigma(35) gene transcripts 4 h ahead of the parent strain, but there was no effect on sigma(28) synthesis. In wild-type B. thuringiensis subsp. israelensis cells, cry4A mRNA was observed from 12 h onwards, while in the mutant it appeared at 8 h and was produced for a longer period. The total amount of cry4A transcripts produced by the mutant was higher than by the parent strain. There was a 60 to 70% reduction in the sporulation efficiency of the mutant B. thuringiensis subsp. israelensis strain compared to the wild-type strain.

The ptsH gene from Bacillus thuringiensis israelensis. Characterization of a new phosphorylation site on the protein HPr

The ptsH gene from Bacillus thuringiensis israelensis (Bti), coding for the phosphocarrier protein HPr of the phosphotransferase system has been cloned and overexpressed in Escherichia coli. Comparison of its primary sequence with other HPr sequences revealed that the conserved His15 and Ser46 residues were shifted by one amino acid and located at positions 14 and 45, respectively. The biological activity of the protein was not affected by this change. When expressed in a Bacillus subtilis ptsH deletion strain, Bti HPr was able to complement the functions of HPr in sugar uptake and glucose catabolite repression of the gnt and iol operons. A modified form of HPr was detected in Bti cells, and also when Bti ptsH was expressed in E. coli or B. subtilis. This modification was identified as phosphorylation, because alkaline phosphatase treatment converted the modified form to unmodified HPr. The phosphoryl bond in the new form of in vivo phosphorylated HPr was resistant to alkali treatment but sensitive to acid treatment, suggesting phosphorylation at a histidine residue. Replacement of His14 with alanine in Bti HPr prevented formation of the new form of phosphorylated HPr. The phosphorylated HPr was stable at 60 degrees C, in contrast with HPr phosphorylated at the N delta 1 position of His14 with phosphoenolpyruvate and enzyme I. (31)P-NMR spectroscopy was used to show that the new form of P-HPr carried the phosphoryl group bound to the N epsilon 2 position of His14 of Bti HPr. Phosphorylation of HPr at the novel site did not occur when Bti HPr was expressed in an enzyme I-deficient B. subtilis strain. In addition, P-(N epsilon 2)His-HPr did not transfer its phosphoryl group to the purified glucose-specific enzyme IIA domain of B. subtilis.

Inorganic phosphate regulates CryIVA protoxin expression in Bacillus thuringiensis israelensis

The role of nutritional factors during CryIVA protoxin expression in Bacillus thuringiensis israelensis (Bti) has been investigated. Inorganic phosphate (Pi) was found to stimulate 135 kD protoxin synthesis by Bti cells. There was a corresponding increase in the cryIVA specific mRNA in the presence of Pi. Inorganic phosphate inhibited HPr kinase but activated HPr phosphatase, the two enzymes responsible for regulating the concentration of phosphorylated HPr in the cell. Addition of protein phosphatase inhibitors NaF and calyculin A during resuspension resulted in the inhibition of toxin synthesis by Bti cells. Calyculin A inhibited HPr phosphatase activity in the in vitro assay also. The concentration of phosphorylated HPr was upregulated when the cells were resuspended in the presence of calyculin A, while the levels of the same were lowered in the presence of Pi, as determined by Western blotting the respective cells. The efficiency of sporulation of Bti was not affected when Pi was added alone or along with the phosphatase inhibitor calyculin A.