Biopolymers: Plastics from Genetically Engineered Organisms November 2010
Scientists at Brookhaven National Laboratory (Upton, New York) and Dow AgroSciences (Indianapolis, Indiana) have genetically engineered a plant species—Arabidopsis—to create the building block for certain plastics, such as polyethylene, that currently derive from fossil fuels. Although plants find use as sources for many biopolymers such as cellulose or starch, they have not been as useful at providing bio-based monomers because the monomers make up such a small proportion of the total biomass, making them relatively expensive to extract. By metabolically engineering plants, scientists can increase production or create entirely novel compounds within the plant. In this case, the scientists produced seeds with high levels of omega-7 fatty acids. The scientists took genes—coding for saturase enzymes—from plants that naturally produce omega-7 fatty acids, such as cat's claw vine and milkweed, whose yield and growth characteristics are not suitable for commercial production. Increasing the efficiency of one saturase enzyme leads to an increase in yields of omega-7 fatty acids from 2% to about 14% by weight, but this yield is still not high enough for industrial-scale production. But simply transplanting these genes into Arabidopsis results in no improvement in yield, so the scientists must also engineer the metabolism of the plant. To do so, scientists down-regulate genes that compete for the introduced enzyme's fatty-acid substrate and also introduce desaturases capable of intercepting substrate that has escaped the first desaturase enzyme as it progresses through the oil-accumulation pathway. This metabolic engineering results in an accumulation of the desired omega-7 fatty acid at levels of about 71% in the best engineered Arabidopsis individuals—much greater than the omega-7 fatty acid levels in milkweed, but equivalent to those in cat's claw vine.
Bio-Based Monomers from Yeast
Researchers at the Polytechnic Institute of New York University (Brooklyn) are engineering yeasts to synthesize omega-hydroxy fatty acids as sources for a unique family of polyethylene-like biobased plastics. This type of fatty acids is difficult and expensive to synthesize using traditional methods, but the scientists have engineered a strain of the yeast Candida tropicalis to produce commercially viable yields of omega-hydroxy fatty acids. The scientists develop the strain by eliminating 16 genes from the C. tropicalis genome that would normally oxidize the key alcohol groups in omega-hydroxy fatty acids. Furthermore, strains expressing other introduced enzymes could convert other fatty acids with different chain lengths and other properties into the desired molecules.