ORIGINS: On the origin of photosynthesis

Where would we be without photosynthesis? In the third essay in Science's series in honor of the Year of Darwin, Mitch Leslie details researchers' efforts to piece together how and when organisms first began to harness light's energy.

An excerpt from the article follows...

Biochemists James Allen and JoAnn Williams of Arizona State University, Tempe, and colleagues are working out how a bacterial reaction center could have evolved photosystem II's appetite for electrons. Taking a hands-on approach, they have been tinkering with the reaction center of the purple bacterium Rhodobacter sphaeroides to determine if they can make it more like photosystem II. First they targeted bacterio- chlorophyll, the bacterial version of chlorophyll that's at the core of the reaction center, and altered the number of hydrogen bonds. Adding hydrogen bonds hiked the molecule's greed for electrons, they found. The water-cleaving portion of photosystem II sports four manganese atoms that become oxidized, or lose electrons. So the team equipped the bacterial reaction center with one atom of the metal. In this modified version, the added manganese also underwent oxidation, the researchers reported in 2005. James Allen says that their creations aren't powerful enough to split water. But eventually, they hope to engineer a reaction center that can oxidize less possessive molecules, such as hydrogen peroxide, that would have been present on the early Earth. Even if the researchers never replicate photosystem II, "if we define the intermediate stages, we've accomplished a lot," he says.