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Published online on February 2, 2007, 10.1073/pnas.0611035104

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Microbiology
Light-powering Escherichia coli with proteorhodopsin

( light-driven proton pumps | oceanic bacteria )

Jessica M. Walter *{dagger}, Derek Greenfield *{ddagger}, Carlos Bustamante *{dagger}{ddagger}{sect}¶||, and Jan Liphardt *{dagger}{ddagger}**

Departments of *Physics, {sect}Chemistry, and Molecular and Cell Biology, ||Howard Hughes Medical Institute, and {ddagger}Biophysics Graduate Group, University of California, Berkeley, CA 94720; and {dagger}Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Contributed by Carlos Bustamante, December 13, 2006 (sent for review December 1, 2006)

Proteorhodopsin (PR) is a light-powered proton pump identified by community sequencing of ocean samples. Previous studies have established the ecological distribution and enzymatic activity of PR, but its role in powering cells and participation in ocean energy fluxes remains unclear. Here, we show that when cellular respiration is inhibited by depleting oxygen or by the respiratory poison azide, Escherichia coli cells expressing PR become light-powered. Illumination of these cells with light coinciding with PR's absorption spectrum creates a proton motive force (pmf) that turns the flagellar motor, yielding cells that swim when illuminated with green light. By measuring the pmf of individual illuminated cells, we quantify the coupling between light-driven and respiratory proton currents, estimate the Michaelis-Menten constant (Km) of PR (103 photons per second/nm2), and show that light-driven pumping by PR can fully replace respiration as a cellular energy source in some environmental conditions. Moreover, sunlight-illuminated PR+ cells are less sensitive to azide than PR- cells, consistent with PR+ cells possessing an alternative means of maintaining cellular pmf and, thus, viability. Proteorhodopsin allows Escherichia coli cells to withstand environmental respiration challenges by harvesting light energy.

Author contributions: J.M.W. designed research; J.M.W. performed research; J.M.W., D.G., and J.L. wrote the paper; D.G. analyzed data; D.G. performed model simulations; and C.B. and J.L. conceived the experiment.

The authors declare no conflict of interest.

**To whom correspondence should be addressed.

Jan Liphardt, E-mail: liphardt{at}physics.berkeley.edu

www.pnas.org/cgi/doi/10.1073/pnas.0611035104
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