Nitrogen-fixing cyanobacteria as gene delivery system for expressing mosquitocidal toxins of Bacillus thuringiensis ssp. israelensis

Classical biological control is the most successful and promising way to replace chemical pesticides. The subspecies israelensis of Bacillus thuringiensis (Bti) is a safe and efficient agent to control mosquito larvae and hence mosquito-borne diseases. One approach to overcome the low efficacy and short half-life in nature of current formulations of Bti is by expressing the toxin genes in recombinant cyanobacteria as a delivery system. Attempts to express Bti toxin in cyanobacteria have been carried out during the last ten years. Toxicities of the transgenic strains were however very low, even under regulation of strong promoters, too low to be effective in vivo. Two Bti Cry proteins have recently been co-expressed in the filamentous nitrogen-fixing cyanobacterium Anabaena PCC 7120, resulting in clones with the highest toxicities and stabilities ever reached so far. However, to obtain a long-lasting preparation, it would be useful to express Bti toxin genes in cyanobacterial strains isolated from nature. This approach requires development of a system for effective transformation into such strains. Releasing such recombinant strains to open environments is still a major obstacle in exploiting this biotechnology.