Subsequent studies established that proteorhodopsin is a light-driven proton pump, able to transport protons across cellular membranes in order to create stored electrochemical energy. In this respect, it is similar to another protein, bacteriorhodopsin, that's used by bacteria in salt ponds to supplement respiration. However, in experiments in which marine bacteria endowed with proteorhodopsin were exposed to light, there was no response. This begged the question: What does proteorhodopsin actually do?
A recent study out of the University of Kalmar in Sweden, led by Jarone Pinhassi, showed that light could be used to stimulate the growth of some types of marine bacteria carrying proteorhodopsin. This indicated that such bacteria can use a form of photosynthesis to supplement respiration as an energy source, but the extent to which light could be used to replace respiration was still unknown.
"Our thinking was that if you had a system that could harvest energy from two different sources and you knocked out one of those sources then you would probably maximize the alternative energy source," Liphardt said. "Think of it like a capacitor. If a capacitor is already fully charged and you connect a battery to it nothing happens. However, if you drain the capacitor and then connect a battery, a current will flow."
To observe proteorhodopsin in action and measure its effects, Liphardt and his co-authors genetically engineered a strain of Escherichia coli that would express the light-sensitive protein.
Said Walter, "The energy metabolism of E. coli is well understood so it served as an excellent testbed for observing proteorhodopsin activity when the mi
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Source:DOE/Lawrence Berkeley National Laboratory