Will Propane-Making Bacteria Revolutionize Energy?
Alex B. Berezow

Propane is the fossil fuel of red-blooded Americans. What poolside or tailgating experience would be complete without firing up the gas grill and torching some meat? (I know, I know... there are charcoal devotees out there.) Even metropolitan mass transit systems are getting in on the excitement. Fleets of buses that run on "LPG" (liquefied petroleum gas) are burning a mixture of propane and butane.

Currently, propane is extracted from natural gas or crude oil. But, in the long run, this is neither a sustainable nor an environmentally friendly practice. Burning propane extracted from the earth is also not carbon-neutral, though it is better than combusting oil or coal. Thus, researchers are looking for ways to produce renewable "fossil fuels" through the use of alternative technologies, such as synthetic biology. Last year, for instance, scientists engineered E. coli to churn out a biofuel that resembled gasoline.

A team of researchers led by Patrik Jones of the University of Turku in Finland, however, believes they have invented a better method. Instead of gasoline, they focused on propane. Why? Because propane has the distinct advantage of being easily converted from gas to liquid (and back). Thus, as the bacteria release gaseous propane, the authors argue it should be easy to capture and store it as a liquid. Additionally, because propane does not build up in the culture medium, it will not inhibit the growth of the bacteria.

To create their tiny fuel manufacturers, the authors had to design a propane biosynthetic pathway that does not exist in E. coli. To do so, they borrowed genes from multiple bacteria: Bacteroides fragilis, Mycobacterium marinum, Bacillus subtilis, and Prochlorococcus marinus. (The pathway they created is shown below. Black letters denote chemical compounds, while red letters denote enzymes and proteins.)

The pathway shows that their genetically engineered E. coli can be fed glucose (sugar) and crank out propane. The next step is scaling up for large-scale industrial production. Their current platform is inadequte for that, but some preliminary data suggests reason for optimism. They would also like to transfer the pathway into a photosynthetic microbe, which would essentially convert the energy of sunlight into propane.

There are three big lessons from this research:

First, it is time for anti-GMO activists to grow up and accept biotechnology. The future of everything from food and medicine to renewable energy and high-tech materials will rely on it.

Second, the potential of synthetic biology (recently discussed in the journal Nature Reviews Molecular Cell Biology) may be limited merely by our imagination. Already, synthetic biology has been used by bioengineers to create genetic logic gates -- a useful technology for biosensors and biological computers -- and by systems biologists to further decipher the complexity of the cell.

Finally, the benefit of basic research is unforeseeable. Most likely, the scientists who figured out how these obscure metabolic pathways worked never became famous nor expected their research to change the world. And yet it just might.

Source: Pauli Kallio, Andras Pasztor, Kati Thiel, M. Kalim Akhtar, & Patrik R. Jones. "An engineered pathway for the biosynthesis of renewable propane." Nature Communications. Published 2-Sep-2014. DOI: 10.1038/ncomms5731

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