Nancy Ho on Engineering the Saccharomyces Yeast
to Produce Cellulose Ethanol

 

At the end of the 1970s, there was a worldwide concerted effort to genetically engineer the Saccharomyces yeast to ferment xylose to ethanol using recombinant DNA techniques.  By then, LORRE had already established a great reputation in developing various technologies required for the conversion of cellulosic biomass to ethanol.  LORRE naturally wanted to pursue the development of recombinant yeast to ferment xylose.  This provided an opportunity for Nancy Ho to be involved in the development of ethanol fuel from cellulosic biomass.

 

After Dr. Ho completed her Ph.D. at Purdue University, she stayed at Purdue for personal reasons.  Although she did not vigorously pursue a high-ranking academic position, she always thrived on solving important problems in science and technology.  For example, two years after she received her Ph.D. from Purdue, she began to intensely study the development of a new method for fragmenting the structure of DNA – facilitating the analysis of its sequence.  Back then DNA was still a “black hole” in molecular biology, with no suitable means to delineate its intricate structure.   Her very first proposal for that project (also the very first formal proposal she ever submitted) received great reviews from the three agencies the proposal was submitted to – NIH, NSF, and the National Cancer Institute.  They all awarded her grants.  From that time on, she supported herself as well as the scientists working with her at Purdue University. 

 

As a scientist, Dr. Ho’ s greatest strength has been her ability to critique her own work and appreciate other scientists’ talent and innovation.  After several years of hard work, the method that she was developing for determining DNA structure was proceeding well.  However, additional restriction enzymes were discovered by then.  She foresaw that restriction enzymes were far superior for tackling the DNA structure.  As such, she gave up her own pursuit and vigorously studied restriction enzyme-based recombinant DNA techniques.  Within a year or so, she became the so-called “expert for recombinant DNA technology” at Purdue.  When Purdue’s Laboratory of Renewable Resources Engineering (LORRE) needed a specialist to help them shape their strategy for developing a recombinant microorganism to ferment biomass, she was recommended to Dr. George Tsao, the Director of LORRE.  Once Dr. Tsao contacted her, she was intrigued by the project because it was an ideal case where the application of recombinant DNA technology could make a real difference.  Furthermore, if successful, she believed that such engineered yeast could help to solve the world’s energy and economic problems in the future.  She decided to join LORRE to lead the study.

 

In the early 1980s, there were about ten groups independently pursuing the task of genetically engineering the Saccharomyces yeast to ferment xylose and more than half of the groups were in the US.  Both the USDA (The United States Department of Agriculture) and the DOE (the Department of Energy) National Laboratories had at least one group pursuing this goal as well.  Not only was her group the smallest, but Dr. Ho also had to obtain most of her own funding – through grant applications to the Federal Government Agencies – to support her group’s research.

 

In short, after encountering serious setbacks in earlier attempts to engineer the yeast, all other US groups (including those of the USDA and the DOE National Laboratories) had given up on this pursuit by the middle 1980s.   Most experts concluded that it might be impossible to engineer the Saccharomyces yeast to ferment xylose.  In addition, after the oil crisis eased in the mid-1980s, it became far more difficult to obtain funding for research in alternative fuels. By the end of the 1980s, there were only four groups – Dr. Ho’s group at Purdue University, two groups in Europe, and one group in Japan – still pursuing this important project. Dr. Ho continued her work, because she felt that according to her analysis and design, the Saccharomyces yeast could be genetically engineered to ferment xylose. She also wanted to exhaust all possibilities before giving up because the Saccharomyces yeast is the safest and most effective microorganism for the conversion of sugars to ethanol. Furthermore, she felt very strongly that cellulosic biomass should be converted to ethanol or other chemicals. Despite these obstacles, Dr. Ho continued this project with great determination. In the long run, her persistence paid off.

 

In 1993, Dr. Ho’s Group at Purdue succeeded in the development of the world’s first genetically engineered yeast that could effectively ferment xylose AND co-ferment both glucose and xylose to ethanol.  This was accomplished by cloning three genes, XR, XD and XK, which are crucial for converting xylose to ethanol, into a small circular DNA molecule known as a plasmid by recombinant DNA techniques.  The recombinant plasmid was then transferred into the host yeast.  The other three groups in Europe and Japan cloned only two genes, XR and XD (but not XK). As a result, their work was not successful and their yeast was unable to ferment xylose to ethanol.

 

Since 1993 to the present, her group has continued to improve the Saccharomyces yeast to more cost effectively produce cellulosic ethanol on an industrial scale. This will be described under the Technology section.