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 (⌬p) and increased the rate of electron transport. However, removal of the ionophore, which caused an increase in ⌬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 ⌬p.The purple nonsulfur phototroph Rhodobacter sphaeroides is a motile, chemotactic bacterium capable of growth and division under a wide range of conditions, including aerobic heterotrophic, anaerobic phototrophic, anaerobic photoheterotrophic, and dark anaerobic conditions, using an alternative electron acceptor like dimethyl sulfoxide (DMSO) or trimethylamine-N-oxide. To grow and divide under these different conditions, R. sphaeroides can induce a range of alternative oxidases and terminal reductases, including one for DMSO and trimethylamine-N-oxide. Not all of the different carriers and terminal reductases are induced to high levels under all conditions, and there appears to be some redundancy. For example, during anaerobic growth in the dark using DMSO or trimethylamine-N-oxide as an electron acceptor, R. sphaeroides synthesizes a full complement of photosynthetic pigments, as well as the ''alternative'' cytochrome b 260 oxidase (38), and during anaerobic photoheterotrophic growth, both the b 260 and b 410 (KCN-sensitive) oxidases are induced (22,39). This redundancy allows R. sphaeroides populations to respond rapidly, in terms of electron transport, to changes in their environment.Previous work on the phenomenon of aerotaxis in Escherichia coli and Salmonella typhimurium has indicated that it is independent of the methyl-accepting chemotaxis proteins used as part of the primary sensing system in these organisms (26, 34), although methyl-accepting chemotaxis proteins have been implicated in aerotaxis in Bacillus subtilis (37). R. sphaeroides appears to have a chemotaxis system comprising both methylaccepting chemotaxis protein-dependent and -independent pathways (36). This, in conjunction with its flexible electron transport system, makes R. sphaeroides an interesting organism for the study of taxis to effectors linked to electron transport. Aerotaxis and phototaxis in the purple photosynthetic bacteria were first identifie...

Rhodobacter sphaeroides responds to a decrease in light intensity by a transient stop followed by adaptation. There is no measurable response to increases in light intensity. We confirmed that photosynthetic electron transport is essential for a photoresponse, as (i) inhibitors of photosynthetic electron transport inhibit photoresponses, (ii) electron transport to oxidases in the presence of oxygen reduces the photoresponse, and (iii) the magnitude of the response is dependent on the photopigment content of the cells. The photoresponses of cells grown in high light, which have lower concentrations of light-harvesting photopigment and reaction centers, saturated at much higher light intensities than the photoresponses of cells grown in low light, which have high concentrations of light-harvesting pigments and reaction centers. We examined whether the primary sensory signal from the photosynthetic electron transport chain was a change in the electrochemical proton gradient or a change in the rate of electron transport itself (probably reflecting redox sensing). R. sphaeroides showed no response to the addition of the proton ionophore carbonyl cyanide 4-trifluoromethoxyphenylhydrazone, which decreased the electrochemical proton gradient, although a behavioral response was seen to a reduction in light intensity that caused an equivalent reduction in proton gradient. These results strongly suggest that (i) the photosynthetic apparatus is the primary photoreceptor, (ii) the primary signal is generated by a change in the rate of electron transport, (iii) the change in the electrochemical proton gradient is not the primary photosensory signal, and (iv) stimuli affecting electron transport rates integrate via the electron transport chain.

The behavioural response of Rhodobacter sphaemides to temporal changes in the concentration of chemoeffectors, and to stimuli affecting electron transport, was analysed using tethered cells. Populations of photosynthetically grown tethered cells of R. sphaemides showed a transient response, a stop followed by adaptation, to a stepwise reduction in the concentration of chemoattractants (such as organic acids or sugars) and terminal electron acceptors. A step-down response was also measured in free swimming ceHs to a reduction in light intensity. As this response appears to apply to all effectors this suggests that there is a sensory pathway in anaerobically grown R. sphaemides which responds primarily to a reduction in a stimulus. R. sphaemides therefore responds when moving down a concentration gradient. This is the inverse of the classical Escherichie coli-2almonella typhimurium model of bacterial sensory behaviour in which bacteria respond primarily when there is an increase in an attractant concentration, i.e. when moving up a gradient. R. sphaemides does show a chemokinetic response to an increase in concentration of a limited number of compounds but this response is sustained and accompanied by an increase in the rate of flagellar rotation and therefore not simply equivalent to the transient increase in smooth swimming measured in E. d i on addition of an attractant.

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