Bioengineering a microbial workforce

 Xiao Qiu and colleagues at the University of Saskatchewan have engineered a yeast to produce high yields of ricinoleic acid, which occurs naturally in castor beans. U.S. DEPARTMENT OF AGRICULTURE

They may not punch timecards, wear protective clothing or gossip around the water cooler, but microorganisms are a vital part of the industrial work force. The metabolic pathways of microorganisms have been harnessed and engineered to produce molecules that are important to the pharmaceutical, food, chemical and alternative-energy industries. In the November issue of the Journal of Lipid Research, investigators from the University of Saskatchewan describe how they engineered one particular yeast, Pichia pastoris, to produce ricinoleic acid, a rare fatty acid commonly used in the manufacture of a wide variety of products.

Ricinoleic acid is produced naturally in high amounts by castor beans. But harvesting the acid from the beans can be tricky, since castor beans also produce high levels of the potent toxin ricin. To circumvent this problem, researchers have tried identifying other ways of procuring ricinoleic acid from plants. Initially, tobacco and the commonly used plant model organism Arabidopsis were engineered to express an enzyme of the ricinoleic acid biosynthesis pathway. But both plants produced a comparatively small fraction of ricinoleic acid and proved to be poor replacements for the castor bean.

Dr. Qiu and his colleagues have a long-standing interest in lipid biosynthesis and bioengineering, and they turned to microorganisms to tackle the ricinoleic acid problem. Like the castor bean, the fungus Claviceps purpurea is known to produce high levels of ricinoleic acid. Dr. Qiu previously had identified two enzymes, CpFAH and CpDGAT2, that were important for the biosynthesis of ricinoleic acid in C. purpurea and had demonstrated that when these enzymes were expressed in yeast, the yeast produced higher levels of ricinoleic acid.

In the article, Dr. Qiu and his colleagues describe how they took this production process one step further by identifying yet another enzyme that plays a critical role in the biosynthesis of ricinoleic acid. When expressed in yeast, the enzyme CpDGAT1 significantly outperformed CpDGAT2. With this new enzymatic ace in hand, they turned to the yeast Pichia pastoris, which is known for producing a high yield of biomass and oil. They incorporated CpFAH and CpDGAT1 into the genome in such a way that they could express these enzymes conditionally. After inducing expression for three days, ricinoleic acid accounted for more than half of the total fatty acids, representing a significant improvement over the production levels seen in engineered plants.

While Dr. Qiu’s work represents a substantial step forward in bioengineering microorganisms for industrial gains, it does raise some interesting questions. For instance, why was Pichia a more effective organism for ricinoleic acid? Can Pichia be engineered to produce high amounts of other important biomaterials? Furthermore, can we determine what makes Pichia such a robust organism and then engineer a plant to have the same capabilities? However these questions are answered, it has become clear that microorganisms can make powerful contributions to modern industry and society.

Kathleen McCann Kathleen McCann earned her Ph.D. in genetics from Yale University. She is now a postdoctoral fellow at the National Institute of Environmental Health Sciences.