In a push to find better biofuels to reduce gasoline consumption and lower greenhouse-gas emissions, scientists have genetically engineered E. coli that is highly efficient in producing butanol, a promising new type of biofuel. The new technology could speed up the development of butanol biofuels into a cost-effective alternative to ethanol.
While ethanol is the main biofuel on the market today, energy firms are increasingly looking to alternatives such as butanol. “It has many attractive properties,” says Jim McMillan, manager of biorefining process R&D at the National Renewable Energy Laboratory’s National Bioenergy Center, in Golden, CO. Because butanol packs more energy per gallon than ethanol does, cars running on butanol get better mileage. And, unlike ethanol, it doesn’t mix with water, so it can be shipped in existing petroleum pipelines without causing problems.
A number of research groups are engineering microbes that can convert sugar from various feedstocks into butanol. Most of these groups rely on the bacterium Clostridium acetobutylicum, which naturally makes a form of butanol called 1-butanol. “But Clostridium is not easy to deal with,” says James Liao, a chemical engineer at the University of California, Los Angeles. “It grows slowly, it’s very fastidious, and it’s not easy to genetically manipulate.” Despite decades of tinkering by scientists, the microbe still can’t produce enough butanol to make it economically viable as a transportation fuel, Liao says.
Instead, he and his colleagues turned to E. coli. Although the bacterium does not produce butanol naturally, it is easy to modify and grows fast. Instead of tweaking the pathway that the microbes employ for fermenting sugar into alcohol, Liao reasoned that he could program E. coli to produce butanol by diverting some of the microorganism’s metabolites into alcohol production. These metabolites, called keto acids, are involved in the synthesis of amino acids, the building blocks of proteins.
To make butanol from keto acids, the researchers inserted two different nonnative genes into E. coli. The first gene came from a microbe commonly used in the production of cheese. The gene codes for an enzyme that converts keto acids into aldehydes. The second gene, derived from yeast, codes for an enzyme that converts aldehydes into butanol.
Initially, when linked together in E. coli, the two genes allowed the microbe to produce small amounts of butanol. With further genetic modifications, Liao was able to dramatically increase the efficiency of the process. For instance, deleting certain genes and boosting the activity of others increased the amount of keto acids available for conversion into butanol. With all the combined manipulations, the engineered microbes achieved an efficiency high enough for industrial use, says Liao.