Cellulose is amongst the most abundant biomass on the planet. And for good reason. It’s sturdy, and it’s simple. The dry weight of an oak tree is mostly cellulose, for instance. As any one knows who’s ever cooked s’mores by a fire, there’s a lot of energy in that cellulosic biomass. An active area of research is to convert cellulose into simple sugars and then ferment those sugars into fuels like ethanol and biodiesel.
The only problem is that cellulose isn’t easily biodegraded. Consider the spanish galleons still sitting in the bottom of the mediterranean – the wooden hulls completely undigested by the myriad bacteria and algae swimming around them – they have lasted hundreds of years without biodegradation.
Biofuel producers can add enzymes and chemicals to more rapidly degrade cellulose and make it available to bacteria to make fuel, but this process is currently too expensive to yield economically viable cellulosic biofuels.
To expedite this process, researchers recently synthesized a new strain of the bacteria E. coli that can digest cellulose, and ferment it into the fuels we desire. Turning this one organism into a one stop shop for biofuel production. The schematic (depicted below) from their recent PNAS paper below depicts how a multifunctional bacterium could streamline the biofuel production process.
In order to produce this “swiss-army” bacterium, the authors had to make a few biological quantum leaps. First, they needed to pick an enzyme that could degrade cellulose and xylose (the two major complex sugars locked up in switchgrass and other cellulosic feed stocks). But that’s not all. This enzyme also needed to be successfully exported from E. coli, which do not typically secrete protein very well. To do this, the authors fused cellulose- and xylose- degrading enzymes to a protein known to be secreted by E. coli and tested which fusion enzymes worked best to degrade the starches of interest. After testing enzymes from a variety of organisms, the authors settled on the cellulase enzyme found in Baccillus sp. D04 and the xylase enzyme found in Clostridium stercorarium.
Next, the authors needed to find a way for the bacteria to express the right amount of enzyme at the right time to achieve optimal growth. One strategy might be to induce expression of genes under a chemical promoter that could be controlled from the outside. That process sounded expensive and hard to regulate. Instead, the authors opted for finding the best endogenous promoters to express their new enzymes exactly when the bacteria needed them. To do this, the authors screened a series of promoters already present in E. coli. By taking this strategy, the authors got the bacteria to grow as well on cellulose as they can on glucose! not a small task at all.
Finally, now that the authors had the sugar-degrading machinery in place, they needed to put in the fuel-producing machinery. Fortunately, the authors had already engineered genetic constructs containing sets of enzymes for the production of butanol (a gasoline substitute), Pinene (a jet fuel substitute) and Fatty acid Ethyl Ester (a biodiesel). It’s always good to stand on the shoulders of your own research!
The resulting “swiss-army bug” is depicted below.
Link to the recent PNAS article -> http://www.ncbi.nlm.nih.gov/pubmed/22123987