Team Boosts Microbial Diesel Fuel Production

jay Keasling and Fuzhong Zhang

Jay Keasling and researchers with the Joint BioEnergy Institute (JBEI) have developed a new technique that boosts microbial production of renewable biodiesel fuel as much as threefold. They call this new technique a dynamic sensor-regulator system or (DSRS).

"DSRS is the first synthetic system that can dynamically regulate a metabolic pathway and improve production of fatty acid-based fuels while the microbes are in the bioreactor," says Jay Keasling, co-author of a paper in Nature Biotechnology: "Design of a dynamic sensor-regulator system for production of [fatty-acid]-based chemicals and fuels."

In the past, fatty acid-based fuel production has been hampered during synthesis because earlier methods only provided static control of gene expression levels.

"When a gene expression control system is tuned for a particular condition in the bioreactor and the conditions change, the control system will not be able to respond and product synthesis will suffer as a result," Fuzhong Zhang, a co-author of the study, says.

Developing new biosensors and promoters

DSRS responds to the on-going metabolic status of the microbe in the bioreactor by sensing key intermediate metabolites in an engineered pathway; then the DSRS regulates intermediates to improve the pathway as conditions change in the bioreactor.

To create their DSRS, Zhang, Keasling and Carothers focused on a strain of Escherichia coli (E. coli) bacteria engineered at JBEI to produce diesel fuel directly from glucose. In this latest work, the JBEI researchers first developed biosensors for a key intermediate metabolite - fatty acyl-CoA - in the diesel pathway. They then developed a set of promoters (segments of DNA) that boost the expression of specific genes in response to cellular acyl-CoA levels. These synthetic promoters only become fully activated when both fatty acids and the inducer reagent known as "IPTG" are present.

"For a tightly regulated metabolic pathway to maximize product yields, it is essential that leaky gene expressions from promoters be eliminated," Zhang says. "Since our hybrid promoters are repressed until induced by IPTG and tuned automatically by the FA/acyl-CoA level, they can be readily used to regulate production of biodiesel and other fatty acid-based chemicals."

Introducing the DSRS improved the stability of this

E. coli strain and tripled the yield of fuel, reaching 28 percent of the theoretical maximum. Keasling feels that with further refinements yields should go even higher. Perhaps more importantly, DSRS should also work with other chemical products, both fatty acid-based and beyond.

"Given the large number of natural sensors available, our DSRS strategy can be extended to many other biosynthetic pathways to balance metabolism, increase product titers and yields, and stabilize production hosts," Zhang says. "It should one day be possible to dynamically regulate any metabolic pathway, regardless of whether a natural sensor is available or not, to make microbial production of commodity chemicals and fuels competitive on a commercial scale."

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Photo: Keasling and Zhang, Roy Kaltschmidt, Berkeley Lab;
Graphics: Ecoli, JEBI; Fatty acid production,