Yeast Technology for Cellulosic Ethanol

Chronological outline of the development of the ideal glucose/xylose

co-fermenting Saccharomyces yeast (cellulosic ethanol-producing yeast) by

Dr. Nancy Ho’s Group at Purdue University
 

 1985-1993         Successfully developed the technology to genetically engineer any Saccharomyces (baker’s) yeast to effectively co-ferment glucose and xylose to ethanol.  This was accomplished by cloning three highly modified xylose-metabolizing-genes, XR, XD and XK, cloned on a high-copy-number plasmid, followed by transforming the yeast with the plasmid (incorporating the plasmid into the yeast cells).  A high-copy-number plasmid is a plasmid capable of self-replicating in the host cells many times. As a result, the host cells will contain many copies of the cloned genes via the plasmids. 
 

1993-1996         Successfully developed the stable engineered yeast with cloned genes integrated into the yeast chromosome.  Genetically engineered yeast containing genes cloned on plasmids are not stable and not suitable for large-scale industrial production of ethanol.  Thus, Dr. Ho’s group had to develop stable yeast with many copies of the same three genes integrated (inserted) into the chromosomes of the yeast.  By then, there was no suitable method for this genetic task.  As such, Dr. Ho developed a new method for incorporating genes into the yeast chromosomes in high-copy numbers.  This new method is easy to perform and extremely reliable.  It can transform any yeast, including industrial yeasts containing two or more sets of chromosomes, into stable engineered yeast containing numerous copies of the cloned genes inserted into the yeast chromosomes.  Their first successful genetically engineered stable yeast was 1400(LNH-ST), with multiple copies of the three genes XR-XD-XK integrated together as a cassette into the chromosome of the yeast strain 1400.   
 

1997-1999         Large-scale screening for better yeasts with no legal constraints for converting cellulosic sugars (mixed sugars recovered from cellulosic biomass) to ethanol.  Although 1400 (LNH-ST) was already sufficiently effective for industry to use for the production of cellulosic ethanol, it might not be optimal.  Thus Dr. Ho decided to screen yeast strains that were effective for converting glucose to ethanol, make them able to ferment xylose with her technologies, and select the best among them for industry to produce cellulosic ethanol.  Furthermore, the selected yeasts for screening were all free from legal constraints.  Among the yeasts tested and integrated with the XR-XD-XK genes (more than ten yeast strains), 424A (LNH-ST) and 259A (LNH-ST) are effective for industrial production of cellulosic ethanol.
 

2000-Present     Further genetic engineering of the best yeast, 424A (LNH-ST), to improve its xylose fermentation and to make it ferment two other minor sugars effectively.  There are at least three separate tasks that need to be done to improve the yeast’s production of cellulosic ethanol.  The further improved yeast should be able to ferment xylose and other minor sugars 30% to 50% faster. 
 

2002-present     Successful engineering of yeast capable of producing high-value co-products during ethanol production.  One drawback to the production of ethanol, including grain ethanol, is that the profit margin is very narrow.  In the overall strategy for the development of recombinant yeast for the efficient conversion of cellulosic biomass to ethanol, Dr. Ho planned to make the yeast capable of producing high-value co-products with ethanol production.  This will allow ethanol production to be far more cost-effective and profitable.  In the past two years, the Ho group at Purdue University has made their recombinant glucose/xylose co-fermenting yeasts produce two important industrial products as co-products for either grain-ethanol or cellulosic-ethanol production.  Production of co-products with the production of ethanol can improve the profit for ethanol producers from $0.25 to $1.00 per gallon.  The current most urgent task of the Ho group at Purdue University is to secure funding and quickly engineer additional co-products into their yeast so that ethanol producers (including current grain-ethanol producers) can generate different co-products for extra profit and further lower the ethanol price. Producing co-products with ethanol production would not only aid the ethanol industry, farmers, and the public, but it would also benefit other industries that need such products to thrive.  The detergent, pulp and paper, and animal feed industries will all benefit from the first generation of co-products that the Ho yeasts are made to produce.

Dr. Ho’s recombinant glucose/xylose co-fermenting yeasts contain several additional unique features.  The engineered yeasts are robust industrial yeast, not laboratory strains.  These recombinant yeasts were made industrial-user friendly, and can be used immediately without further development.  The engineered yeast was made environmentally friendly as well; it does not require the use of toxic and expensive chemicals such as antibiotics to maintain the plasmids containing the cloned genes in the yeast.  These were all accomplished through careful and ingenious design, driven by the desire to provide our country (as well as the world) with an ideal means to convert our largest renewable resource, cellulosic biomass, to ethanol fuel or other green chemicals.

Dr. Ho has led the world in this field since 1993.  Her work has been appreciated worldwide, which is evidenced by the international awards bestowed on Dr. Ho (see awards and industrial endorsements).  Her work was also widely reported by newspapers and magazines in the US and around world.