"So instead of taking a piece from here and from there and stitching them together, we can just take this part of the gene," Cann explains. "You can cut this and put it into another microbe."
On the surface of the cell, there is an enzyme that cuts the xylan into small pieces and a protein that binds to the pieces and brings them inside the cell. Enzymes within the cell metabolize the sugar.
The reason this microbe, unlike most others used to make biofuels, is able to degrade xylan is that it has evolved an enzyme that allows it to remove the side chains, or decorations, that are part of xylan's structure. They hinder the degradation process by preventing complete accessibility of the enzymes to the sugar chain.
Once the side chains have been removed, another enzyme in the microbe breaks the sugar chain down into single sugars, or xylose. Other enzymes within the cell then metabolize the xylose.
Having the enzymes next to each other on the genome is convenient for scientists who are working on engineering microbes that can degrade both cellulose and hemicellulose. The cluster could be designed as a cassette and put into a microbe that normally degrades only cellulose.
Moreover, being next to each other allows them to work efficiently. "You have a set of enzymes that have co-evolved," Cann explains. "If they have co-evolved over millions of years, it means they have been fine-tuned to work together."
Another advantage of Caldanaerobius polysaccharolyticus is that it is a thermophilic bacterium, and its enzymes are resistant to temperatures as high as 70 degrees Celsius. Biofuel fermentation is usually done at 37 degrees Celsius, a temperature at which most microbes can survive. This means that the material in the fermentation vats is easily contaminated.
The next step for Cann and his collaborators is to develop techniques for transferring this gene cluster, which is quite large, into microbes.
The research was recently published in the Journal of Biological Chemistry and is available here. Yejun Han, Vinayak Agarwal, Dylan Dodd, Jason Kim, Brian Bae, and Satish K. Nair are co-authors.
The Energy Biosciences Institute is a four-partner research collaboration that includes the University of Illinois, the University of California at Berkeley, Lawrence Berkeley National Laboratory, and BP, the energy company that funds the work. It is dedicated to applying the biological sciences to the challenges of producing sustainable, renewable energy for the world.