THE CHEMOKINETIC AND CHEMOTACTIC BEHAVIOR OF RHODOBACTER-SPHAEROIDES - 2 INDEPENDENT RESPONSES

Rhodobacter sphaeroides exhibits two behavioral responses when exposed to some compounds: (i) a chemotactic response that results in accumul ation and (ii) a sustained increase in swimming speed. This latter che mokinetic response occurs without any apparent long-term change in the size of the electrochemical proton gradient. The results presented he re show that the chemokinetic response is separate from the chemotacti c response, although some compounds can induce both responses. Compoun ds that caused only chemokinesis induced a sustained increase in the r ate of flagellar rotation, but chemoeffectors which were also chemotac tic Caused an additional short-term change in both the stopping freque ncy and the duration of stops and runs. The response to a change in ch emoattractant concentration was a transient increase in the stopping f requency when the concentration was reduced, with adaptation taking be tween 10 and 60 s. There was also a decrease in the stopping frequency when the concentration was increased, but adaptation took up to 60 mi n. The nature and duration of both the chemotactic and chemokinetic re sponses were concentration dependent. Weak organic acids elicited the strongest chemokinetic responses, and although many also caused chemot axis, there were conditions under which chemokinesis occurred in the a bsence of chemotaxis. The transportable succinate analog malonate caus ed chemokinesis but not chemotaxis, as did acetate when added to a mut ant able to transport but not grow on acetate. Chemokinesis also occur red after incubation with arsenate, conditions under which chemotaxis was lost, indicating that phosphorylation at some level may have a rol e in chemotaxis. Aspartate was the only chemoattractant amino acid to cause chemokinesis. Glutamate caused chemotaxis but not chemokinesis. These data suggest that (i) chemotaxis and chemokinesis are separate r esponses, (ii) metabolism is required for chemotaxis but not chemokine sis, (iii) a reduction in chemoattractant concentration may cause the major chemotactic signal, and (iv) a specific transport pathway(s) may be involved in chemokinetic signalling in R. sphaeroides.