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 the total cell protein. Live cell bioassays in dH₂0 demonstrated toxicity of the cry4Aa₇₀₀and 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.