Swimming changes and chemotactic responses in Rhodobacter sphaeroides do not involve changes in the steady state membrane potential or respiratory electron transport

The effect of the chemoattractants acetate, propionate, pyruvate and potassium on the steady state membrane potential and the rate of respiratory electron transport was examined in Rhodobacter sphaeroides. Under conditions where the membrane potential makes up most of the proton motive force, all attractants increased the steady state membrane potential in the dark. However, only potassium was able to consistently increase the steady state membrane potential in the light. All of the attractants were able to increase the mean swimming speed of cells at high light levels for prolonged periods, showing that the increase in mean speed cannot be caused by an increase in the membrane potential. Measurement of chemotaxis in wells showed a positive response to propionate in the dark, at low light (10 - 20 μmol photons m-2 s-1) and in high light (700 μmol photons m-2 s-1). The demonstration of chemotaxis in the dark precluded any direct role of photosynthetic electron transport in chemotaxis. The response at high light, where there was no induced change in membrane potential, confirmed that the steady state membrane potential was not involved in tactic signalling. Acetate, propionate and pyruvate at appropriate concentrations stimulated the rate of respiratory electron transport in the dark, while potassium had no effect. In low light, all three organic acids caused a significant stimulation of respiratory electron transport but potassium caused a significant inhibition. In high light, only pyruvate and propionate caused a significant increase in the rate of respiratory electron transport. Chemoattractants can therefore produce a significant positive tactic response when respiratory electron transport is either unaffected or inhibited. These data show that neither a change in the bulk steady state membrane potential nor the rate of respiratory electron transport causes either the change in swimming behaviour or acts as a chemotactic signal. © 1990 Springer-Verlag.