Motile bacteria possess a stimulus-response system in which the input is the local concentration of an attractant or repellent and the output is the motion of the cell. It is known that some chemicals, e.g., sugars and amino acids, are sensed by specific receptors and that changes in receptor occupancy lead to changes in flagellar rotational bias (the probability that the flagella spin counterclockwise), but the way in which the receptors are coupled to the flagella remains obscure. We will continue our studies of the physiology of this system by examining the rotational behavior of tethered cells of Escherichia coli exposed to step and impulse stimuli. From responses of genetically engineered strains containing modified tar (aspartate receptor) and tsr (serine receptor) proteins, we expect to learn how information about receptor occupancy crosses the cytoplasmic membrane and how different receptor classes interact. From the responses of flagellar markers on filamentous cells stimulated at different points along their length, we will determine the physical properties of the substance that carries information from the receptors to the flagella. We should be able to determine its diffusion coefficient and its size. By tracking and tethering cells from the same culture and comparing their swimming and rotational behavior, we expect to learn more about synchronization of different flagella in the same cell. This work complements that on genetics and biochemistry pursued in other laboratories. The long-range goal is an understanding at the molecular level of basic processes involved in receptor function and sensory transduction.