Electron transport-dependent taxis in Rhodobacter sphaeroides

Rhodobacter sphaeroides showed chemotaxis to the terminal electron acceptors oxygen and dimethyl sulfoxide, and the responses to these effectors were shown to be influenced by the relative activities of the different electron transport pathways. R. sphaeroides cells tethered by their flagella showed a step-down response to a decrease in the oxygen or dimethyl sulfoxide concentration when using them as terminal acceptors. Bacteria using photosynthetic electron transport, however, showed a step-down response to oxygen addition. Addition of the proton ionophore carbonyl cyanide 4-trifluoromethoxyphenylhydrazone did not cause a transient behavioral response, although it decreased the electrochemical proton gradient (delta p) and increased the rate of electron transport. However, removal of the ionophore, which caused an increase in delta p and a decrease in the electron transport rate, resulted in a step-down response. Together, these data suggest that behavioral responses of R. sphaeroides to electron transport effectors are caused by changes in the rate of electron transport rather than changes in delta p.