BOSTON, Mass. (Mar. 4, 2010) Reduce. Reuse. Recycle. We hear this mantratime and again. When it comes to carbon‹the "Most Wanted" element in termsof climate change‹nature has got reuse and recycle covered. However, it's upto us to reduce. Scientists at Harvard Medical School are trying to meetthis challenge by learning more about the carbon cycle, that is, the processby which carbon moves from the atmosphere into plants, oceans, soils, theearth's crust, and back into the atmosphere again.
One of the biggest movers and shakers is the lowly cyanobacteria, anocean-dwelling, one-celled organism. Pamela Silver, HMS professor of systemsbiology, and colleagues have uncovered details about how this bacteriafixes, or digests, carbon. These bacteria build miniature factories insidethemselves that turn carbon into fuel.
Silver and her colleagues report that the bacteria organize these factoriesspatially, revealing a structural sophistication not often seen insingle-celled organisms. This regular and predictable spacing improves theefficiency of carbon processing. In the future, an understanding of themechanisms that govern this spatial organization may help improve theefficiency of designer bacteria engineered to produce carbon-neutral fuelssuch as biodiesel and hydrogen.
These findings will be published online March 5 in the journal Science.
The rod-shaped cyanobacteria are among the most abundant organisms on earth.Forty percent of the carbon in the carbon cycle is reused and recycledthrough these tiny creatures. To process carbon, cyanobacteria buildsoccer-ball-shaped structures inside themselves called carboxysomes. Thesetiny factories absorb carbon dioxide and convert it into sugar, which thebacteria then use to produce energy.
"The ocean is just packed with these bacteria. By studying them, we'reunderstanding more about how the earth works," said Silver, who is also onthe faculty of the Wyss Institute for Biologically Inspired Engineering atHMS. "I'm blown away by what's happening in the ocean and what we don'tunderstand about it. There are a lot of things in the ocean that are goingto be useful to us."
The research team, led by co-first authors, research fellows David Savageand Bruno Afonso, attached a fluorescent tag to proteins involved inbuilding the carboxysome, then grew the tagged bacteria under a microscope.
The resulting images revealed that, instead of being randomly numbered andhaphazardly placed, cyanobacteria build carboxysomes in numbers that scalewith their size, and they space the factories evenly along their length.(see image, end of release)
The finding adds evidence for new ways to think about bacteria. "We had thisidea of bacteria as a bag of enzymes, but that has been completelyshattered," said Afonso.A single protein, called parA, acts as a kind of inner-bacterium stagemanager, arranging the carboxysomes in a neat, single-file row, theresearchers found. When they disabled the bacteria's ability to make theprotein, the carboxysomes were distributed far more randomly.
The cyanobacteria lacking parA were also less "fit" for survival, saidSavage. While wild-type bacteria cells have a consistent number ofcarboyxsomes, which in turn optimizes carbon processing and fitness, theknockout bacterium created daughter cells whose numbers of carboxysomesranged from none to an excess. The daughter cells with few or nocarboxysomes divide more slowly and also process fifty percent less carbonthan daughter cells at the other end of the spectrum. (see video 1)
By tagging parA in wild-type bacteria, they discovered interesting dynamicsin the protein. Thousands of parA proteins repeatedly cluster together andshoot quickly from one end of the bacterium to the other. (see video 2)
"It's amazing that you can generate this regularity and symmetry potentiallyfrom a single protein," said Savage. "It's amazing that it is somehow tunedby the dynamics of the protein." The researchers have not yet identified theexact mechanism parA uses to govern the spacing.
Many other bacteria also have the parA protein, which is known forseparating chromosomes during cell division. "This work highlights howbacteria cobble together spare parts to achieve similar goals such asorganization and segregation," said David Rudner, HMS assistant professor ofmicrobiology and molecular genetics, who was not involved in the study.
These findings may help synthetic biologists one day create designerbacteria.
"Knowledge about how cells create and deploy specialized factories like thecarboxysome opens the way to creating other kinds of mini factories thatcould perform useful functions," said Richard Losick, Harvard Universityprofessor of molecular and cellular biology, who was not involved in thestudy.
Silver's lab is looking into whether the carboxysome might be useful foroptimizing the production of hydrogen by engineered bacteria. One challengein designing hydrogen-producing bacteria is that the enzymes that producehydrogen are sensitive to oxygen. The carboxysome may help solve thisproblem because its outer shell blocks out oxygen, protecting the enzymesinside from its toxic effects.
Source: Harvard Medical School