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Newly engineered yeast can simultaneously ferment glucose and xylose to produce ethanol; significant cost benefits in ethanol production

A collaborative led by researchers at the University of Illinois and including the Lawrence Berkeley National Laboratory, the University of California at Berkeley, Seoul National University and BP have engineered a strain of Saccharomyces cerevisiae—the common industrial yeast—to co-ferment glucose and xylose simultaneously to produce ethanol.

In a process of adjustments to the original yeast, Yong-Su Jin at the University of Illinois and his colleagues converted it to one that will consume both types of sugar faster and more efficiently than any strain currently in use in the biofuel industry. The new yeast strain simultaneously converts cellobiose (a precursor of glucose) and xylose to ethanol just as quickly as it can ferment either sugar alone.

Glucose is a six-carbon sugar that is relatively easy to ferment; xylose is a five-carbon sugar that has been much more difficult to utilize in ethanol production. The new strain, made by combining, optimizing and adding to earlier advances, reduces or eliminates several major inefficiencies associated with current biofuel production methods.

To overcome these bottlenecks, we engineered yeasts to co-ferment mixtures of xylose and cellobiose. In these yeast strains, hydrolysis of cellobiose takes place inside yeast cells through the action of an intracellular ß-glucosidase following import by a high-affinity cellodextrin transporter. Intracellular hydrolysis of cellobiose minimizes glucose repression of xylose fermentation allowing co-consumption of cellobiose and xylose.

The resulting yeast strains, co-fermented cellobiose and xylose simultaneously and exhibited improved ethanol yield when compared to fermentation with either cellobiose or xylose as sole carbon sources. We also observed improved yields and productivities from co-fermentation experiments performed with simulated cellulosic hydrolyzates, suggesting this is a promising co-fermentation strategy for cellulosic biofuel production. The successful integration of cellobiose and xylose fermentation pathways in yeast is a critical step towards enabling economic biofuel production.

—Ha et al.

The new yeast strain is at least 20% more efficient at converting xylose to ethanol than other strains, making it the best xylose-fermenting strain reported in any study, according to Jin.

The Energy Biosciences Institute, a BP-funded initiative, supported the research. A paper on their work was published in the Proceedings of the National Academy of Sciences (PNAS).

S. cerevisiae has been used for centuries in baking and brewing because it efficiently ferments sugars and in the process produces ethanol and carbon dioxide. The biofuel industry uses this yeast to convert plant sugars to bioethanol. While S. cerevisiae is very good at utilizing glucose, a building block of cellulose and the primary sugar in plants, it cannot use xylose, a secondary but significant component of the lignocellulose that makes up plant stems and leaves. Most yeast strains that are engineered to metabolize xylose do so very slowly.

Xylose is a wood sugar, a five-carbon sugar that is very abundant in lignocellulosic biomass but not in our food. Most yeast cannot ferment xylose.

—Yong-Su Jin

Yeasts altered to take up xylose is that they will suck up all the glucose in a mixture before they will touch the xylose, Jin said. A glucose transporter on the surface of the yeast prefers to bind to glucose. The yeast’s extremely slow metabolism of xylose also adds significantly to the cost of biofuels production.

If you do the fermentation by using only cellobiose or xylose, it takes 48 hours. But if you do the co-fermentation with the cellobiose and xylose, double the amount of sugar is consumed in the same amount of time and produces more than double the amount of ethanol. It’s a huge synergistic effect of co-fermentation.

—Suk-Jin Ha, U of I postdoc and lead author

The team first gave the yeast a cellobiose transporter. Cellobiose, a part of plant cell walls, consists of two glucose sugars linked together. Cellobiose is traditionally converted to glucose outside the yeast cell before entering the cell through glucose transporters for conversion to ethanol. Having a cellobiose transporter means that the engineered yeast can bring cellobiose directly into the cell. Only after the cellobiose is inside the cell is it converted to glucose.

This approach, initially developed by co-corresponding author Jamie Cate at the Lawrence Berkeley National Laboratory and the University of California at Berkeley, eliminates the costly step of adding a cellobiose-degrading enzyme to the lignocellulose mixture before the yeast consumes it.

It has the added advantage of circumventing the yeast’s own preference for glucose. Because the glucose can now “sneak” into the yeast in the form of cellobiose, the glucose transporters can focus on drawing xylose into the cell instead. Cate worked with Jonathan Galazka, of UC Berkeley, to clone the transporter and enzyme used in the new strain.

The team then tackled the problems associated with xylose metabolism. The researchers inserted three genes into S. cerevisiae from a xylose-consuming yeast, Picchia stipitis.

Graduate student Soo Rin Kim at the University of Illinois identified a bottleneck in this metabolic pathway, however. By adjusting the relative production of these enzymes, the researchers eliminated the bottleneck and boosted the speed and efficiency of xylose metabolism in the new strain.

They also engineered an artificial isoenzyme that balanced the proportion of two important cofactors so that the accumulation of xylitol, a byproduct in the xylose assimilitary pathway, could be minimized. Finally, the team used evolutionary engineering to optimize the new strain’s ability to utilize xylose.

The cost benefits of this advance in co-fermentation are very significant, Jin said.

We don’t have to do two separate fermentations. We can do it all in one pot. And the yield is even higher than the industry standard. We are pretty sure that this research can be commercialized very soon.

—Yong-Su Jin

Jin noted that the research was the result of a successful collaboration among principal investigators in the Energy Biosciences Institute and a BP scientist, Xiaomin Yang, who played a key role in developing the co-fermentation concept and coordinating the collaboration.


  • Suk-Jin Ha, Jonathan M. Galazka, Soo Rin Kim, Jin-Ho Choi, Xiaomin Yang, Jin-Ho Seo, N. Louise Glass, Jamie H. D. Cate, and Yong-Su Jin (2010) Engineered Saccharomyces cerevisiae capable of simultaneous cellobiose and xylose fermentation PNAS doi: 10.1073/pnas.1010456108



This is all great and everything...but when will someone produce ethanol at a competitive price without subsidies?

They've had since the early 70's to get that fixed so call me when you have something to talk about.


DaveD, don't forget the fact that the oil industry is heavily subsidized thru tax breaks.


Let's phase out all subsidies and let that market system people keep talking about have a chance. I would phase out corn ethanol and phase in cellulose ethanol. Subsidies may be necessary to get an industry started but sooner or later they have to run on their own.

The price of corn got bid up and the ethanol producers got squeezed. Market people would say then the ethanol producers would not buy and the price would go down. They have sunk costs and must produce or go out of business. The simplistic supply demand people usually leave out elasticity.


danm, I TOTALLY agree and despise the subisdies that the oil industry gets. But I would like to see the subsidies on ethanol ended as well.
If they still need the subsidies after nearly years, then ethanol is not viable for our long term needs.

We DEFINITELY need to end all subsidies to oil and even make them start paying for any military presence we keep in the middle east. Maybe then ethanol could compete without the subisidy?

Everyone keeps telling me that we need cheap oil for our economy. I keep trying to explain to them that it is NOT cheap. They just don't realize how much they pay for it even with their income taxes. As much as $$$1.03TRILLION in 2010 alone. No, not all of the military is for oil, but it sure as hell doesn't take more than a fraction of that to "provide for the common defense" which is what the constitution mandates.

Of course, now I just opened up a can of worms and everyone will start hating on me. Oh well. :-)


Some things are cost shifted in our economy. It is like the $5 band aid to pay for emergency ward people that can not pay... false accounting.

Giving oil companies billions of dollars in subsidies so they will keep the oil coming is one of those as well. They will bring in oil because they make a ton of money, no need for subsidies.

I like cellulose biofuels because the farmer can make more money from the crops that they already grow. If they make more per acre, maybe we can reduce some subsidies there as well.


BTW, cheap oil is like cheap labor, you become dependent on it. It gets factored in and you can not do without it. Not a good idea to cost shift, hide costs or continue to subsidize. The REAL cost of anything should be apparent so that we can make informed decisions.


SJC, EXACTLY! If we actually accounted for the cost of things, we would behave much differently.


DaveD, i misinterpreted your 1st comment. You clarified it quite well and I couldn't agree more.


We do not have cheap fuel in Europe, and we get by.
It costs e1.40 / litre for gasoline at present (in Ireland) and more in Germany. (say $6.80 / US gallon).

And people get by, mainly by driving smaller, more economical cars (and not petrol powered pick-up trucks).

Anyway, oil is now $91 / barrel, so the price is heating up in the US anyway.


Good points DaveD. You could have added the cost of fixing (all) related pollution damages such as increased health care cost, reduced productivity, cost of cleaning up the mess they normally leave behind, climate change oncoming cost, ground and water pollution, etc.

If all those cost were added to crude oil price, the price per barrel may may well over $100. That may give cellulosic ethanol a better chance.


Harvey, You're right, I should have mentioned all those things....but it seemed like piling on! :-)

I guess it's ok to kick an opponent when they're down as long as they deserve it LOL


With $100 per barrel oil near and no end in sight to price hikes, it may be time for alternatives. If we can get rid of OPEC oil, less money goes out of the U.S. to them and we can have a more secure energy future.

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