Fig. 3.

The response of PR⁺ bacteria to green light increases with respiratory inhibition and light intensity. (a) Benefit of illumination versus degree of inhibition of the respiratory system. The difference between angular velocity in green light and red light (ω_G–ω_R) becomes pronounced in PR⁺ bacteria (filled circles) as respiration is inhibited by low oxygen or sodium azide. PR⁻ cells (open circles) show no change between red and green illumination. To facilitate comparison between cells, the angular velocities are normalized by each bacterium's maximum velocity. n = 5–14 cells per condition. Green line, fit to model described in d. (b) The rotation speed of PR⁺ cells depends on the intensity of green illumination. Individual PR⁺ spinner cells were exposed to six intensities of green light. The mean angular velocity at each intensity is plotted (n = 5–6 cells for each intensity), normalized by the velocity at maximum illumination. Dashed lines, fits to model described in d. (c) Overview of transmembrane fluxes and proton pumping in PR⁺ E. coli. Sources of proton motive force include respiration and PR. Sinks include rotation of the flagellar motor and ATP synthesis. (d) Model including sources of pmf (respiration and proteorhodopsin), sinks (such as the flagellar motor), and the membrane capacitance. The variable resistors R_R and R_PR model the effect of azide and light on proton extrusion by respiration and PR, respectively. The voltmeter (top-most circuit element) measures the potential difference across the membrane (equivalent to the pmf).