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"This is the first revelation of how a bacterium
chooses from its more than 100 enzymes to break down a particular
biomass," says David H. Wu, professor in the Department of Chemical
Engineering at the University of Rochester. "Once we know how a
bacterium targets a particular type of biomass, we should be able to
boost that process to draw ethanol from biomass far more efficiently
that we can today."
Ethanol holds the promise of a clean, renewable
alternative to fossil fuels, but deriving it from plants is difficult.
Producing it from corn is the easiest method, but doing so on a large
scale would drive up the price of corn, corn starch, and even
tangential foods like beef, since cows are fed on corn - not to
mention all the energy spent fertilizing, maintaining, and harvesting
a crop like corn. Conversely, deriving ethanol from plant materials
such as the corn stalks and wood chips is challenging because the
plants’ cellulose is a very tough substance to break down, making for
an inefficient process.
Wu’s technique may prove much more effective than
traditional methods. Instead of using separate steps to break down
biomass into glucose and ferment the glucose into ethanol, as is
currently done, Wu is working on a way to make a bacterium break down
and ferment plant biomass efficiently in just one step.
Wu investigated C. thermocellum, which is a
microorganism that has that ability to turn biomass into ethanol in
one step, but is not used at the industrial scale yet because the
first step, breaking down the plant’s cellulose, is much too
inefficient. The key, Wu surmised, is to find out what enzymes the
bacterium uses to accomplish its feat, and then boost its ability to
produce those enzymes. The problem, however, lies in the fact that C.
thermocellum uses more than 100 enzymes, and any of the millions of
combinations of them may be the magic mixture to break down a
particular biomass.
So, Wu decided to make the bacterium do the work
for him.
"The bacteria know how to express just the right
genes to break down any particular biomass substrate, and we wanted to
know how they know to turn on and off just the right genes at the
right time to do the trick," says Wu. "We found the bacterium
essentially throws the whole bowl of spaghetti at the wall, sees what
sticks, and then makes a lot of that particular noodle."
C. thermocelllum produces low levels of many of its
enzymes at any one time. When the bacterium comes in contact with wood,
for instance, a few of its enzymes break down some of that wood. A
product of that tiny reaction is a sugar called laminaribiose that
diffuses into the cell. There it deactivates a repressor for two
genes, which wake up and start pumping out the two triggers the full
production of wood-degrading enzymes CelC and LicA.
Wu’s paper shows the first time the triggering
pathway for enzyme production in this bacterium has been revealed, and
it was only possible because C. thermocellum genome was just recently
sequenced, thanks to Wu’s collaboration with the U. S. Department of
Energy. With its 100 busy enzymes, the entire genome had to be
observed as a whole, since fiddling with combinations of two, three,
or more enzymes at a time would have taken "more than our lifetime,"
Wu says.
Wu is now working to re-engineer C. thermocellum to
express an abundance of particular genes so it can readily and
efficiently produce ethanol from a particular biomass. He’s also
continuing the genome-wide search for enzyme combinations that will
degrade and ferment grasses, corn stovers, and even food waste.
"I don’t think this is the revolution that makes
ethanol a mainstay," says Wu, "but I believe this is a part of what
will lead to the revolution." This research, also authored by Wu’s
graduate students Michael Newcomb and Chun-Yu Chen, is funded by the
U. S. Department of Energy. |
