Factors regulating cryIVB expression in the cyanobacterium--Synechococcus PCC 7942

Expression of a mosquito larvicidal gene in chloroplast and nuclear compartments of Chlamydomonas reinhardtii

As a part of the search for environment-friendly biocontrol of mosquito-borne diseases, mosquito larvicidal potential of Bacillus thuringiensis subsp. jegathesan (Btj) Cry toxins is explored for toxins with increased toxicity. Safe delivery of the Cry toxins to mosquito larvae in aquatic habitats is a major concern. This is because in water bodies Bacillus thuringiensis (Bt) protein formulations degrade by sunlight, can sink down and get adsorbed by the silt. So, because of its short persistence the toxin requires repeated applications at the given site. Therefore, an upcoming approach is incorporating the Bt toxins in Chlamydomonas reinhardtii (C. reinhardtii) because it is a food of mosquito larvae in water and its molecular toolkit is well investigated for foreign gene expression. The present work aimed to compare the feasibility of C. reinhardtii chloroplast and nuclear compartments for stable expression of Cry11Ba toxin as this is the most toxic Btj protein to date, lethal to different mosquito species. With chloroplast expression of cry11Ba gene we were able to generate marker-free C. reinhardtii strain stably expressing Cry11Ba protein and demonstrating mortality against Aedes aegypti larvae. Moreover, for nuclear expression linking the cry11Ba gene to zeocin via foot and mouth disease virus (FMDV) 2A peptide resulted in the selection of transformants with increased cry11Ba mRNA expression levels by semi-quantitative reverse transcriptase PCR. Obtained results lay a foundation for the C. reinhardtii chloroplast expression system to be used for genetic engineering with Bt toxins which possess enhanced toxicity^.

Expression of a Synthetic Gene for the Major Cytotoxin (Cyt1Aa) of Bacillus thuringiensis subsp. israelensis in the Chloroplast of Wild-Type Chlamydomonas

Chlamydomonas reinhardtii (Chlamydomonas) strains that are toxic to mosquito larvae because they express chloroplast transgenes that are based on the mosquitocidal proteins of Bacillus thuringiensis subsp. israelensis (Bti) could be very useful in mosquito control. Chlamydomonas has several advantages for this approach, including genetic controls not generally available with industrial algae. The Bti toxin is produced by sporulating bacteria and has been used for mosquito control for >30 years without creating highly resistant mosquito populations. The suite of toxins is four main proteins: three Cry proteins and the cytotoxic Cyt1Aa (27 kDa). Cyt1Aa is not very toxic to mosquitoes by itself, but it prevents the development of resistance. The production of Cyt1Aa in other microbes, however, has been challenging due to its affinity for certain membrane phospholipids. Here we report on the production of recombinant Cyt1Aa (rCyt1A) in the chloroplast of photosynthetic Chlamydomonas at levels of at least 0.3% total protein. Live cell bioassays demonstrated toxicity of the rCyt1Aa Chlamydomonas to larvae of Aedes aegypti. We also expressed the chloroplast cyt1Aa gene in a wild-type Chlamydomonas strain (21 gr) that can grow on nitrate. These results have implications for developing a Chlamydomonas strain that will be toxic to mosquito larvae but will not induce strongly resistant populations.

Toward mosquito control with a green alga: Expression of Cry toxins of Bacillus thuringiensis subsp. israelensis (Bti) in the chloroplast of Chlamydomonas

We are developing Chlamydomonas strains that can be used for safe and sustainable control of mosquitoes, because they produce proteins from Bacillus thuringiensis subsp. israelensis (Bti) in the chloroplast. Chlamydomonas has a number of advantages for this approach, including genetic controls that are not generally available with industrial algae. The Bti toxin has been used for mosquito control for > 30 years and does not engender resistance; it contains three Cry proteins, Cry4Aa (135 kDa), Cry4Ba (128 kDa) and Cry11Aa (72 kDa), and Cyt1Aa (25 kDa). To express the Cry proteins in the chloroplast, the three genes were resynthesized and cry4Aa was truncated to the first 700 amino acids (cry4Aa₇₀₀ ); also, since they can be toxic to host cells, the inducible Cyc6:Nac2-psbD expression system was used. Western blots of total protein from the chloroplast transformants showed accumulation of the intact polypeptides, and the relative expression level was Cry11Aa > Cry4Aa₇₀₀ > Cry4Ba. Quantitative western blots with purified Cry11Aa as a standard showed that Cry11Aa accumulated to 0.35% of total cell protein. Live cell bioassays in dH₂0 demonstrated toxicity of the cry4Aa₇₀₀ and cry11Aa transformants to larvae of Aedes aegypti and Culex quinquefasciatus. These results demonstrate that the Cry proteins that are most toxic to Aedes and Culex mosquitoes, Cry4Aa and Cry11Aa, can be successfully expressed in the chloroplast of Chlamydomonas.

Poly(A)-assisted RNA decay and modulators of RNA stability

In Escherichia coli, RNA degradation is orchestrated by the degradosome with the assistance of complementary pathways and regulatory cofactors described in this chapter. They control the stability of each transcript and regulate the expression of many genes involved in environmental adaptation. The poly(A)-dependent degradation machinery has diverse functions such as the degradation of decay intermediates generated by endoribonucleases, the control of the stability of regulatory non coding RNAs (ncRNAs) and the quality control of stable RNA. The metabolism of poly(A) and mechanism of poly(A)-assisted degradation are beginning to be understood. Regulatory factors, exemplified by RraA and RraB, control the decay rates of subsets of transcripts by binding to RNase E, in contrast to regulatory ncRNAs which, assisted by Hfq, target RNase E to specific transcripts. Destabilization is often consecutive to the translational inactivation of mRNA. However, there are examples where RNA degradation is the primary regulatory step.

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.

Cyanobacterial toxins as allelochemicals with potential applications as algaecides, herbicides and insecticides

Cyanobacteria ("blue-green algae") from marine and freshwater habitats are known to produce a diverse array of toxic or otherwise bioactive metabolites. However, the functional role of the vast majority of these compounds, particularly in terms of the physiology and ecology of the cyanobacteria that produce them, remains largely unknown. A limited number of studies have suggested that some of the compounds may have ecological roles as allelochemicals, specifically including compounds that may inhibit competing sympatric macrophytes, algae and microbes. These allelochemicals may also play a role in defense against potential predators and grazers, particularly aquatic invertebrates and their larvae. This review will discuss the existing evidence for the allelochemical roles of cyanobacterial toxins, as well as the potential for development and application of these compounds as algaecides, herbicides and insecticides, and specifically present relevant results from investigations into toxins of cyanobacteria from the Florida Everglades and associated waterways.

A cryptic Bacillus isolate exhibited narrow 16S rRNA gene sequence divergence with Bacillus thuringiensis and showed low maintenance requirements in hyper-osmotic complex substrate cultivations

A cryptic Bacillus (K90) isolate obtained from soil samples from the Kuwait desert exhibited lower maintenance requirements in complex substrate cultivations than Bacillus thuringiensis. A mathematical model was used to estimate apparent maintenance coefficients (m(c)) and these were found to be 0.336 and 0.041/h for B. thuringiensis and K90, respectively. The results also showed that the values of apparent maintenance coefficients were inversely related to the specific growth rates. Furthermore, 16S rRNA gene sequencing showed that K90 exhibited 99.81% sequence similarity to that of B. mojavensis and 92.9% with B. thuringiensis. It is evident from the dendrogram that the evolution of B. mojavensis (K90) (B. subtilis group), which may have originated after B. licheniformis could have been influenced by prolonged hyper-osmotic conditions, while B. thuringiensis that evolved before B. oleronius exhibited greater sensitivity as implied by the higher maintenance coefficient obtained for the hyper-osmotic cultures. As K90 exhibited low maintenance requirements in hyperosmotic cultures, close phylogenetic relationship with B. thuringiensis, along with the reported property of encapsulation of insecticidal crystal proteins (Cry) in Bacillus strains and endophytic nature of B. mojavensis, strongly suggest that K90 could be a promising surrogate host for the transgenic delivery of "Cry" proteins.

Mosquito larvicidal activity of transgenic Anabaena PCC 7120 expressing toxin genes from Bacillus thuringiensis subsp. israelensis

Genes encoding the mosquito larvicidal toxins Cry4Aa, Cry11Aa, Cyt1Aa and the regulatory P20 from Bacillus thuringiensis subsp. israelensis were introduced into the nitrogen-fixing, filamentous cyanobacterium Anabaena PCC 7120 for expression under control of two strong promoters P(psbA) and P(A1). The clone pRVE4-ADRC displayed toxicity against fourth-instar larvae of Aedes aegypti, the highest ever achieved in cyanobacteria. It was about 2.5-fold more toxic than the respective clone without cyt1Aa [Wu et al., Appl. Environ. Microbiol. 63 (1997) 4971-4975]. Cyt1Aa synergized the combination of Crys by about five-fold. Consistently, the lethal times exerted by pRVE4-ADRC were also reduced (it killed exposed larvae more quickly). This clone may become a useful biological control agent which reduces the probability of resistance development in the target organisms [Wirth et al., Proc. Natl. Acad. Sci. USA 94 (1997) 10536-10540].

Mosquito larvicidal activity of transgenic Anabaena strain PCC 7120 expressing combinations of genes from Bacillus thuringiensis subsp. israelensis

Various combinations of the genes cryIVA (cry4A), cryIVD (cry11A), and p20 from Bacillus thuringiensis subsp. israelensis were introduced into the nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120 by means of Escherichia coli-Anabaena shuttle vector pRL488p and were expressed under control of two tandem strong promoters, a cyanobacterial promoter (PpsbA) and an E. coli T7 promoter (PA1). Two of the clones carrying cryIVA plus cryIVD, one with p20 and one without p20, displayed toxicity against third-instar larvae of Aedes aegypti at levels greater than any level previously reported for transgenic cyanobacteria.

Identification and characterization of a gene encoding a vertebrate-type carbonic anhydrase in cyanobacteria

A gene (designated ecaA) encoding a vertebrate-like (alpha-type) carbonic anhydrase (CA) has been isolated from two disparate cyanobacteria, Anabaena sp. strain PCC 7120 and Synechococcus sp. strain PCC 7942. The deduced amino acid sequences correspond to proteins of 29 and 26 kDa, respectively, and revealed significant sequence similarity to human CAI and CAII, as well as Chlamydomonas CAHI, including conservation of most active-site residues identified in the animal enzymes. Structural similarities between the animal and cyanobacterial enzymes extend to the levels of antigenicity, as the Anabaena protein cross-reacts with antisera derived against chicken CAII. Expression of the cyanobacterial ecaA is regulated by CO2 concentration and is highest in cells grown at elevated levels of CO2. Immunogold localization using an antibody derived against the ecaA protein indicated an extracellular location. Preliminary analysis of Synechococcus mutants in which ecaA has been inactivated by insertion of a drug resistance cassette suggests that extracellular carbonic anhydrase plays a role in inorganic-carbon accumulation by maintaining equilibrium levels of CO2 and HCO3- in the periplasm.

Expression of mouse metallothionein in the cyanobacterium Synechococcus PCC7942

A cDNA encoding mouse metallothionein was cloned into the shuttle vector pUc303, creating a translational fusion with the bacterial chloramphenicol acetyltransferase gene. The resulting fusion protein has been expressed in the cyanobacterium Synechococcus PCC7942. Cyanobacterial transformants expressed mouse metallothionein-specific mRNA species as detected by RNA slot blots. In addition, the transformants expressed a unique cadmium ion-binding protein corresponding to the predicted size of the mouse metallothionein fusion protein. Expression of this fusion protein conferred a two- to five-fold increase in cadmium ion tolerance and accumulation on Synechococcus PCC7942.