Our civilisation depends on petrol, but we need to find a way to make it carbon-neutral (Image: OJO Images / Rex)

Take brewer’s yeast, add a gene from a salt marsh plant, grow it with an obscure bacterium found in a French landfill, and what have you got? A cheap, renewable way to fuel our cars, claims Christopher Voigt, a synthetic biologist at the University of California, San Francisco.

While biofuels derived from plants can theoretically be a carbon-neutral energy source, many also displace food-producing crops. Making them from cellulose – structural material abundant in crop waste and grasses – can sidestep that problem.

But efficient processes to do so are lacking. Voigt’s team was looking for a way to get microbes to do the hard work, converting cellulose from crop waste or grasses into chemicals called methyl halides, which can in turn be turned into regular gasoline in a simple catalytic reaction.


Enzyme hunt

A variety of plants and microorganisms naturally make methyl halides in small amounts using methyl halide transferase enzymes (MHTs). But only a handful of such enzymes were known, so Voigt’s team set out on a detective hunt to find more.

They scoured DNA sequence databases for genes that would produce proteins 18% or more similar to the known MHTs. Then they asked a DNA synthesis company to make the 89 matching genes found, and spliced them into the genome of E. coli bacteria, to see which of them produced methyl halides most efficiently.

“We were essentially mining the sequence databases for function,” Voigt explains.

The clear winner was one of the previously known MHT genes, from Batis maritima, known as turtleweed or saltwort, a plant found on the salt marshes of the southeastern US and California.

Bug buddies

Voigt’s team spliced the gene into yeast to produce a strain able to make methyl halides in large amounts. But the puzzle was not over yet. They still needed to find an organism that would digest cellulose into smaller molecules that the yeast could readily convert into the substrate for the MHT enzyme.

Most cellulose-digesting microbes grow slowly, and become efficient only at relatively high temperatures. The researchers needed an organism that could grow at about the same rate as yeast at the same temperature it favours – around 30 °C.

After an extensive search through the scientific literature, they found the ideal candidate: a bacterium called Actinotalea fermentans, isolated in the 1980s from a landfill dump in France.

That bacterium excretes acetate: if it is cultured alone, it soon poisons itself with this waste product. But yeast can happily use acetate as a food source.

Voigt and colleagues had assembled the perfect microbial team – A. fermentans converts cellulose into acetate, which is in turn made into methyl halides by the engineered yeast. It is a low-temperature, cheap process that produces the methyl halides that are readily converted into fuel.

Cheaper than oil

The researchers are now working to make the process more efficient, altering their yeast’s genes to tune its metabolism to produce more substrate for the MHT enzyme from the available acetate. Assuming their system could be made to work as efficiently as yeast converts sugars to ethanol, they calculate that it could produce gasoline more cheaply than from oil.

Voigt’s novel co-culture is one of several attempts to make microbes produce advanced biofuels. For instance, James Liao‘s team at the University of California, Los Angeles, has engineered E. coli to produce long-chain alcohols, which pack more energy than the plant-derived ethanol that is the main biofuel used today.

Meanwhile, South San Francisco company LS9 is tinkering with bacterial biochemical pathways that turn sugars into fatty acids – which can be converted to biodiesel.

“It’s valuable to have as many approaches on the table as possible,” says Jay Keasling of the University of California, Berkeley, who heads the US Department of Energy’s Joint BioEnergy Institute in Emeryville, California.

Journal reference: Journal of the American Chemical Society (DOI: 10.1021.ja8094611u)