Yeast rises to a new occasion

A tiny, one-celled organism that turns agricultural wastes into ethanol may be the key to providing car drivers with an inexpensive, clean-burning fuel.

provided by Purdue University News Service

ancy Ho, a molecular geneticist at Purdue University, has modified the genes of a particular type of yeast so that the tiny organism can convert more of the sugars found in plant matter leftover corn stalks, tree leaves, wood chips, grass clippings, even cardboard boxes into ethanol. Her nearly 20-year effort to produce the genetically engineered yeast has earned her and her industrial partners an R&D 100 Award, to be given Sept. 24 by R&D Magazine to the developers of the year's 100 most technologically significant products and processes.

Ethanol, a form of alcohol, is a liquid fuel that can be used by itself or blended with gasoline to create "gasohol." When burned, ethanol produces far less air pollution and greenhouse gases than gasoline. Currently, ethanol is produced when yeast ferments the glucose, a form of sugar, found in food crops such as cane sugar, corn and other starch-rich grains. However, Ho says, these crops are expensive and in limited supply, making them too costly to produce ethanol on a large scale.

The genetically engineered yeast produces at least 30 percent more ethanol from a given amount of plant material than the unmodified version of the yeast or any other yeast. It is also super-stable and does not need to be grown in special nutrients or under special conditions. And the yeast can use agricultural and other organic wastes an abundant, completely renewable domestic resource rather than food crops, a potential benefit to farmers who could gain extra income by selling crop residues to companies that produce ethanol.

"This plant material is an ideal and inexpensive feedstock for ethanol fuel production," Ho says. "This genetically engineered yeast will make it possible to substantially lower the cost of producing ethanol on a large scale. The goal is to make ethanol not only competitive with the cost of gasoline at the pump, but even much cheaper. Using ethanol produced from plant wastes not only will reduce our country's dependence on foreign oil, but, because it is clean-burning, it also will reduce air pollution and greenhouse gas emissions from cars."

Ho and her colleagues in the Molecular Genetics Group at Purdue's Laboratory of Renewable Resources Engineering (LORRE) won the R&D 100 Award for their work with SWAN Biomass Co., Oakbrook Terrace, Ill., to develop the genetically modified yeast. The annual awards will be given at a Sept. 24 banquet and exhibit at Chicago's Museum of Science and Industry.


Something to wine about


The yeast that Ho modified, called Saccharomyces yeast, is an environmentally safe microorganism commonly used by industry to ferment glucose into ethanol. It also has been used since ancient times to make beer and wine. But glucose is only one type of sugar in plant matter. Beginning in the early 1980s, Ho's research group and others around the world attempted to genetically modify the yeast so that it could ferment both glucose and another plant sugar, xylose, into ethanol. A yeast that could ferment both sugars could produce more ethanol from the same amount of plant material, making the process more economical.

"About 30 to 40 percent of the sugar released from plant matter, called cellulosic biomass, is xylose, and the other 60 to 70 percent is glucose," Ho says. "If you can only ferment 60 percent of the sugar into ethanol, you could never make it cheap enough to use to drive your car." Over several years, all the U.S. research groups gave up pursuing a yeast that could ferment both glucose and xylose, but Ho's group and three other international groups continued.

"I stayed with this project because I thought it was workable, and I was interested in 'green' chemistry," Ho says. "It's a subject that I feel strongly about and that I've always wanted to research something has to be done to solve our waste and pollution problems."


Yeast wearing new genes


In 1993, after more than 10 years of dedicated research, Ho's group became the first in the world to produce a genetically engineered Saccharomyces yeast that can effectively ferment both glucose and xylose. Ho first determined that the Saccharomyces yeast needed to produce three strong enzymes for it to efficiently convert xylose to ethanol. She then selected a yeast that could ferment xylose, but which was not effective for fermenting glucose, and cloned from that yeast three genes responsible for producing the xylose-fermenting enzymes. Ho also developed a new method to integrate multiple copies of the three xylose-metabolizing genes into the chromosome of the Saccharomyces .

"Our genetically engineered yeast not only can effectively ferment xylose, but also can ferment glucose and xylose simultaneously to ethanol, a property that no other natural microorganism has," Ho says. "This is important for the industrial production of ethanol, because it takes less time while producing more product."

In addition, the yeast does not require expensive antibiotics to keep the cloned genes active, and the yeast produces very few byproducts, which would have to be removed from the ethanol.


Call in the clones


Ho also credits the success of the yeast project to LORRE's director, Professor George Tsao, for his foresight nearly 20 years ago in anticipating the importance of gene cloning technology for the future of biotechnology. "In 1980 he decided to establish a molecular genetics research group at LORRE, and he had the courage then to put the responsibility for establishing this group, as well as for carrying out this important project, on a self-made gene-cloning technologist, a woman scientist, and a Purdue graduate who for family reasons had never left Purdue after receiving her Ph.D."

Geneticist Zhengdao Chen and technician Adam Brainard, members of Ho's research group, also contributed to the development of the yeast. In 1993, Amoco Corp. licensed the yeast strain. SWAN Biomass Co., a subsidiary of Amoco, was established to develop the yeast for commercial applications. The company tested the yeast in a large-scale testing facility at the National Renewable Energy Laboratory in Golden, Colo. The results confirmed that the yeast is effective at co-fermenting glucose and xylose from cellulosic biomass into ethanol, Ho says.

Robert Walker, president of SWAN Biomass Co., says, "The first commercial applications of this new technology are likely to be in the production of fuel ethanol from field wastes generated on farms or from wastes that result as crops are processed for sale."

Ho says the next step is to further improve the yeast. "We anticipate that our final engineered yeast strain may require only half the time to co-ferment the same amount of glucose and xylose as the current strain," Ho says. In addition, she says the new method they developed to integrate multiple copies of genes into the yeast chromosome could be used to make yeast capable of producing other high-value byproducts in addition to ethanol.

Ho's research has been funded by the Department of Energy through the Consortium for Plant Biotechnology Research Inc.; the U.S. Environmental Protection Agency; and industry, including SWAN Biomass.

  Sources: Nancy Ho, (765) 494-7046; e-mail, Robert Walker, SWAN Biomass Co., (630) 889-7126. Writer: Amanda Siegfried, (765) 494-4709; e-mail,

Web links: Purdue's Laboratory of Renewable Resources Engineering,; 1998 R&D 100 Award,; R&D Magazine,