The bacterial flagellum is both a motor organelle and a protein export/assembly apparatus. It extends from the cytoplasm to the cell exterior. All the protein subunits of the external elements have to be exported. Export employs a type III pathway, also utilized for secretion of virulence factors. Six of the components of the export apparatus are integral membrane proteins and are believed to be located within the flagellar basal body. Three others are soluble: the ATPase that drives export, a regulator of the ATPase, and a general chaperone. Exported substrates diffuse down a narrow channel in the growing structure and assemble at the distal end, often with the help of a capping structure.

A "temporal gradient apparatus" has been developed that allows the motility of bacteria to be studied after they have been subjected to a sudden change from one uniform concentration of attractant to another. A sudden decrease elicits the tumbling response observed with spatial gradients; it was found, however, that a sudden increase also elicits a response, namely supercoordinated swimming. This demonstrates that chemotaxis is achieved by modulation of the incidence of tumbling both above and below its steady-state value. The initial responses gradually revert to the steady-state motility pattern characteristic of a uniform distribution of attractant. The apparent detection of a spatial gradient by the bacteria therefore involves an actual detection of a temporal gradient experienced as a result of movement through space. Potential models for the chemotactic response based on some "memory" mechanism are discussed.The phenomenon of chemotaxis occurs widely in biological systems. Its presence in bacteria was detected by Pfeffer (1) in 1881, and has been clarified further in recent years, in particular by the recent studies of Adler and his coworkers (2, 3). In many ways bacterial chemotaxis appears analogous to sensory reception in higher species as in (a) the specificity of the response to attractants (2, 4), (b) the indication that the receptor molecules are located in the outer membrane (3, 5), and (c) the sensitivity of the response to ratios of concentrations rather than to differences (1, 6).4 However, bacterial chemotaxis poses a special problem: how can such a small organism detect the concentration differences necessary to sense a gradient in space?The "size problem" in relation to gradient sensing can be readily calculated. In an exponential gradient with a decay distance of 20 mm, the difference in concentration of attractant at the two extremes of a 2-utm long bacterium is only 1 part in 104. Since bacteria respond strongly in such a gradient, an analytical device capable of discerning 1 part in 104 is required if the sensing system simply utilizes spatial separation. A further problem arises in relation to the statistical fluctuations of attractant in the vicinity of the receptors.Assuming hypothetical sampling volumes of 1 am X 1 gm X 0.1 um, near the "head" and "tail" of a bacterium, only 60 molecules of attractant would be present at 1 MM, yet chemotaxis is known to occur at such concentrations. The standard deviation of 60 molecules is 1 /60, showing that statistical fluctuations can be much greater than the needed accuracy.

SummaryWe have examined the cytoplasmic components (FliH, FliI and FliJ) We tested for protein±protein interactions by affinity blotting. In many cases, a given protein interacted with more than one other component, indicating that there are likely to be multiple dynamic interactions or interactions that involve more than two components. Interactions of FlhB C with rod/hook-type substrates were strong, whereas those with filament-type substrates were very weak; this may reflect the role of FlhB in substrate specificity switching. We propose a model for the flagellar export apparatus in which FlhA and FlhB and the other four integral membrane proteins of the apparatus form a complex at the base of the flagellar motor. A soluble complex of at least three proteins (FliH, FliI and FliJ) bind the protein to be exported and then interact with the complex at the motor to deliver the protein, which is then exported in an ATP-dependent process mediated by FliI.

Mutations in the fliK gene of Salmonella typhimurium commonly cause failure to terminate hook assembly and initiate filament assembly (polyhook phenotype). Polyhook mutants give rise to pseudorevertants which are still defective in hook termination but have recovered the ability to assemble filament (polyhook-filament phenotype). The polyhook mutations have been found to be either frameshift or nonsense, resulting in truncation of the C terminus of FliK. Intragenic suppressors of frameshift mutations were found to be ones that restored the original frame (and therefore the C-terminal sequence), but in most cases with substantial loss of natural sequence and sometimes the introduction of artificial sequence; in no cases did intragenic suppression occur when significant disruption remained within

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