Engineering the Perfect Combination: Methyl Halide Production from Renewable Carbon Sources in Engineered Yeast

An attractive method to satisfy the growing demand for fuel is the conversion of non-food agricultural resources into liquid fuels (Bayer et al. 2009). Specifically, methyl halides are useful reactants for gasoline production via a catalyst called Zeolite. Methyl halides show promise as a petroleum substitute because the compounds can be derived from renewable carbon sources. However, the feasibility of producing methyl halides remains a problem. Methyl halides are naturally produced from many organisms, including marine algae, fungi and halophytic plants, but it is time-consuming to harvest them and the yields are low. The enzyme responsible for this process is methyl halide transferase (MHT). Current research is exploring ways to transport MHTs into more industrially sound organisms for faster, more effective methyl halide production. Researchers are now employing a special technique called “synthetic metagenomics” to construct genetic sequences from DNA libraries based on functional similarities. The identified genetic sequences are then cloned into a vector for replication by Escherichia coli (E. coli) or yeast.Alec Faggen 
Bayer, T., Widmaier, D., Temme, K., Mirsky, E., Santi, D., Voigt, C., 2009. Synthesis of Methyl Halides from Biomass Using Engineered Microbes. Journal of The American Chemical Society 131, 65088615.

 Bayer and colleagues working at the University of San Francisco combined naturally producing MHT yeast and cellulolytic bacteria in order to effectively convert lignocellulosic biomass (such as switchgrass, poplar, corn stover, and sugar cane bagasse) to methyl halides. Eighty-nine MHT genes were initially selected after multiple BLAST searches on all putative MHT genes from the NCBI sequence database. These genes came from diverse sources of plants, fungi, bacteria, and unidentified organisms. New chemical synthesis techniques obviated the need for host organisms for cloning, which is especially beneficial because some of these genes are from unknown organisms.
Methyl halide production, using the synthesized MHT genes, was then tested on three ions in E. coli. The MHT from the halophytic plant, B. maritime, presented with the highest activity of all genes on each ion. The B. maritime MHT was then transferred to the yeast S. cerevisiae. Yeast is especially useful as a host organism for its natural resistance to the toxic effects of methyl halides up to high levels. Its productivity from glucose was found to be 12,000 times better than the best production rate from a culturable organism. A co-culture using this newly engineered yeast and the cellulolytic bacterium Actinotalea fermentans attained successful methyl halide production from unprocessed switchgrass (Panicum virgatum), corn stover, sugar cane bagasse, and poplar (Populus sp.). These results demonstrate the potential of producing methyl halides from non-food agricultural resources.

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