ABSTRACT
Behavior patterns of chemotactic mutants of Salmonella typhimurium were compared to those of the wild type by using the quantitative tumble frequency assay. Some cheU mutants were completely inverted in their responses--e.g., attractants produce responses expected for repellents and repellents produce responses expected for attractants. Still others swam smoothly and did not respond to any stimuli. Mutants of other complementation groups were found to exhibit exact additivity or potentiation in response to multiple stimuli whereas the wild type showed desensitization. The results suggest that the cheU gene product acts as a switch at the interface between the sensing system and the motor response. The system is finely tuned so that changes in individual proteins can produce potentiation, desensitization, exact additivity, or inversion of responses.