Influence of transcriptional and translational control sequences on the expression of foreign genes in Caulobacter crescentus

The persistence and ecological impacts of a cyanobacterium genetically engineered to express mosquitocidal Bacillus thuringiensis toxins

BACKGROUND

The cyanobacterium Anabaena PCC 7120#11 has been genetically engineered to act as a delivery vehicle for Bacillus thuringiensis subspecies israelensis mosquitocidal toxins. To address ecological concerns about releasing this genetically engineered microorganism into the environment for mosquito larva control, the persistence and ecological impacts of PCC 7120#11 was evaluated using multi-species, standardized aquatic microcosms.

METHODS

The microcosms were set up as described in ASTM E1366-02 (Standard Practice for Standardized Aquatic Microcosms: Fresh Water), with a few modifications. The treatment group microcosms were inoculated with PCC 7120#11 and key water quality parameters and non-target effects were compared between the treatment and control groups over a period of 35 days.

RESULTS

PCC 7120#11 decreased from a concentration of 4.50 × 10(6) cells/ml (at inoculation) to 1.32 × 10(3) cells/ml after 4 weeks and larvicidal activity against third instar larvae of Anopheles arabiensis was only evident for two weeks after treatment. Both treatment and the interaction of treatment and time had a significant effect on nitrate, phosphate and photosynthetic microorganism concentrations. Treatment with PCC 7120#11 caused a temporary spike in ammonia in the microcosms a week after treatment, but the concentrations were well below acute and chronic criteria values for ammonia in freshwater ecosystems. Cyprinotus vidua concentrations were not significantly different between PCC 7120#11 and control microcosms. In PCC 7120#11 microcosms, Daphnia pulex concentrations were significantly lower than control concentrations between days 18 and 25. By the end of the experiment, none of the measured variables were significantly different between the treatment groups.

CONCLUSIONS

The standard aquatic microcosm experiments provided more data on the ecological impacts of PCC 7120#11 than single-organism assessments would have. On the basis of the relatively minor, short-term effects that PCC 7120#11 had on water quality parameters and non-target invertebrates, further evaluation of PCC 7120#11 for use in integrated vector management is warranted.

Variations in the mosquito larvicidal activities of toxins from Bacillus thuringiensis ssp. israelensis

Comparing activities of purified toxins from Bacillus thuringiensis ssp. israelensis against larvae of seven mosquito species (vectors of tropical diseases) that belong to three genera, gleaned from the literature, disclosed highly significant variations in the levels of LC(50) as well as in the hierarchy of susceptibilities. Similar toxicity comparisons were performed between nine transgenic Gram-negative species, four of which are cyanobacterial, expressing various combinations of cry genes, cyt1Aa and p20, against larvae of four mosquito species as potential agents for biological control. Reasons for inconsistencies are listed and discussed. Standard conditions for toxin isolation and presentation to larvae are sought. A set of lyophilized powders prepared identically from six Escherichia coli clones expressing combinations of four genes displayed toxicities against larvae of three mosquito species, with levels that differed between them but with identical hierarchy.

Co-Expression of the Mosquitocidal Toxins Cyt1Aa and Cry11Aa from Bacillus thuringiensis Subsp. israelensis in Asticcacaulis excentricus

The cyt1Aa gene from Bacillus thuringiensis subsp. israelensis (Bti), whose product synergizes other mosquitocidal toxins, and functions as a repressor of resistance developed by mosquitoes against Bacilli insecticides, was introduced into the aquatic Gram-negative bacterium Asticcacaulis excentricus alongside the cry11Aa gene. The genes were introduced as an operon, but although mRNA was detected for both genes, no Cyt1Aa toxin was detected. Both proteins were expressed using a construct in which a promoter was inserted upstream of each gene. Recombinant A. excentricus expressing both toxins was found to be approximately twice as toxic to third instar larvae of Culex quinquefasciatus as transformants expressing just Cry11Aa.

Expression of the Bacillus thuringiensis Mosquitocidal Toxin Cry11Aa in the Aquatic Bacterium Asticcacaulis excentricus

A mosquitocidal aquatic bacterium has been developed by introducing an operon containing the cry11Aa, and p20 genes from Bacillus thuringiensis subsp. israelensis (Bti) into the gram-negative aquatic bacterium Asticcacaulis excentricus. After transformation, the cry11Aa gene was successfully expressed in recombinant A. excentricus under the tac promoter, at the level of 0.04 pg/cell. The recombinant bacteria were toxic to Aedes aegypti larvae with an LC(50) of 6.83 x 10(5) cells/mL. We believe that these bacteria may have potential as genetically engineered microorganisms for the control of mosquito larvae.

Mosquitocidal toxins, genes and bacteria: the hit squad

Certain entomopathogenic species of bacilli and Clostridium produce one or more toxins that kill mosquito larvae even at concentrations in the picomolar range. Altogether, 19 distinct genes are known that encode mosquitocidal toxins, which vary in their potency, species specificity and mode of action. Unlike chemical insecticides, mosquitocidal bacilli used as larvicides are safe for animals and the environment, and do not affect non-pest insects. Mosquitocidal bacteria are effective to varying degrees against Culex, Anopheles and Aedes mosquito larvae, but their rapid sedimentation from the larval feeding zone, UV-light sensitivity and narrow host range have hampered their development. New genetic engineering approaches are being investigated that could overcome these limitations and allow stable expression of broad host range combinations of toxins in UV-resistant, buoyant recombinant bacteria, as discussed here by Alan Porter.

Expression of mosquito active toxin genes by a Colombian native strain of the gram-negative bacterium Asticcacaulis excentricus

Mosquito control with biological insecticides, such as Bacillus sp. toxins, has been used widely in many countries. However, rapid sedimentation away from the mosquito larvae feeding zone causes a low residual effect. In order to overcome this problem, it has been proposed to clone the Bacillus toxin genes in aquatic bacteria which are able to live in the upper part of the water column. Two strains of Asticcacaulis excentricus were chosen to introduce the B. sphaericus binary toxin gene and B. thuringiensis subsp. medellin cry11Bb gene cloned in suitable vectors. In feeding experiments with these aquatic bacteria, it was shown that Culex quinquefasciatus, Aedes aegypti, and Anopheles albimanus larvae were able to survive on a diet based on this wild bacterium. A. excentricus recombinant strains were able to express both genes, but the recombinant strain expressing the B. sphaericus binary toxin was toxic to mosquito larvae. Crude protease A. excentricus extracts did not degrade the Cry11Bb toxin. The flotability studies indicated that the recombinant A. excentricus strains remained in the upper part of the water column longer than the wild type Bacillus strains.

A broad-host-range expression vector series including a Ptac test plasmid and its application in the expression of the dod gene of Serratia marcescens (coding for ribulose-5-phosphate 3-epimerase) in Pseudomonas stutzeri

A series of expression vectors for gram-negative bacteria was constructed which combine broad-host-range, inducible expression from the tac promoter and diverse antibiotic resistance determinants. The tac promoter activity and the repression by lacIq can be quantitated with a separate test plasmid in the strain to be studied. The dod gene of Serratia marcescens was expressed in Pseudomonas stutzeri and was shown to code for D-ribulose-5-phosphate 3-epimerase.

Improved broad-host-range RK2 vectors useful for high and low regulated gene expression levels in gram-negative bacteria

This report describes the construction and use of improved broad-host-range expression vectors based on the previously constructed pJB137 and pJB653 plasmids (Blatny et al., 1997). These vectors contain the minimal replicon of RK2 and the inducible Pu or Pm promoters together with their regulatory xylR or xylS genes, respectively, from the Pseudomonas putida TOL plasmid pWWO. A set of ATG vectors were derived from pJB653, and these vectors are characterized by the relatively small size, the presence of multiple cloning sites downstream of Pm, the establishment of their nucleotide sequence, the presence of RK2 oriT, and different antibiotic selection markers. The copy numbers of all the vectors can easily be modified by using copy-up mutations of the trfA gene, required for initiation of replication of RK2 replicons. The vectors were used to study the expression levels of the Acetobacter xylinum phosphoglucomutase gene celB and the two commonly used reporter genes luc and cat in Escherichia coli, Pseudomonas aeruginosa, and Xanthomonas campestris. Good induction properties and tight regulation of Pm were achieved in all three species tested, and higher gene expression levels were obtained by using the ATG vectors compared to pJB653. By introducing different trfA copy-up mutations into the vectors, a wide range of gene expression levels from Pu and Pm were obtained in E. coli. Induced expression levels of luc, cat, and celB from Pm were found to be comparable to or higher than those from the Ptrc and PT7 promoters located on high copy number plasmids. The induced levels of Luc activity were higher in P. aeruginosa than in E. coli, indicating that these vectors may be useful for maximization of gene expression in strains other than E. coli. We believe that the well-characterized vectors described here are useful for gene expression studies and routine cloning experiments in many Gram-negative bacteria.

Efficient synthesis of mosquitocidal toxins in Asticcacaulis excentricus demonstrates potential of gram-negative bacteria in mosquito control

The control of mosquitoes with chemical insecticides pollutes the environment and leads to resistance in mosquito populations. Bacterial control of mosquito larvae with Bacillus sphaericus and Bacillus thuringiensis subsp. israelensis, which produce protein toxins, has proved useful, safe, and nonpolluting. These bacteria do, however, suffer from disadvantages, including rapid setting, UV sensitivity, and lack of persistance of spores, proteolysis of toxins, narrow host range, and high production costs. Here we show that the Gram-negative bacterium Asticcacaulis excentricus is a promising host for delivering toxins to mosquito larvae. Plasmid-transformed A. excentricus cells expressing the binary toxin of B. sphaericus exhibited toxicity to Culex and Anopheles mosquito larvae similar to that of the high-toxicity strains of B. sphaericus which produce several toxins. A. excentricus has potential advantages as a larvicide compared with the bacilli, especially persistance in the larval feeding zone, resistance to UV light, lack of toxin-degrading proteases, and low production costs.

Mosquito larvicidal activity of Escherichia coli with combinations of genes from Bacillus thuringiensis subsp. israelensis

The genes cryIVA and cryIVD, encoding 134- and 72-kDa proteins, respectively, and the gene for a regulatory 20-kDa polypeptide of Bacillus thuringiensis subsp. israelensis (serovar H14) were cloned in all seven possible combinations by the Escherichia coli expression vectors pT7 and pUHE. The four combinations containing cryIVA (cryIVA alone, with cryIVD, with the 20-kDa-protein gene, and with both) displayed high levels of mosquito larvicidal activity in pUHE. The toxicity of the combination of cryIVA and cryIVD, with or without the 20-kDa-protein gene, was higher than has ever been achieved with delta-endotoxin genes in recombinant E. coli. Fifty percent lethal concentrations against third-instar Aedes aegypti larvae for these clones decreased (i.e., toxicity increased) continuously to about 3 x 10(5) cells ml-1 after 4 h of induction. Larvicidal activities, obtained after 30 min of induction, were lower for clones in pT7 and decreased for an additional 3.5 h. Induction of either cryIVD or the 20-kDa-protein gene alone resulted in no larvicidal activity in either pT7 or pUHE20. Cloned together, these genes were slightly toxic in pT7 but not in pUHE20. Five minutes of induction of this combination (cryIVD with the 20-kDa-protein gene) in pT7 yielded a maximal mortality of about 40%, which decreased rapidly and disappeared completely after 50 min. CryIVD is thus apparently degraded in E. coli and partially stabilized by the 20-kDa regulatory protein. Larvicidal activity of the combination of cryIVA and cryIVD was sevenfold higher than that of cryIVA alone, probably because of the cross-stabilization of the polypeptides or the synergism between their activities.

Expression of mosquitocidal toxin genes in a gas-vacuolated strain of Ancylobacter aquaticus

A series of plasmids bearing the binary toxin genes of Bacillus sphaericus 2297 or 2317.3, the 100-kDa toxin gene of B. sphaericus SSII-1, or the 130-kDa (cryIVB) toxin gene of Bacillus thuringiensis subsp. israelensis were constructed and introduced into Ancylobacter aquaticus by electroporation. The transformed A. aquaticus cells exhibited significant toxicity towards mosquito larvae, demonstrating a potential use of recombinant A. aquaticus for biological control of mosquitoes.