Phosphorylation-independent bacterial chemoresponses correlate with changes in the cytoplasmic level of fumarate

Abstract

Bacterial chemotaxis is based on modulation of the probability to switch the direction of flagellar rotation. Responses to many stimuli are transduced by a two-component system via reversible phosphorylation of CheY, a small cytoplasmic protein that directly interacts with the switch complex at the flagellar motor. We found that the chemorepellents indole and benzoate induce motor switching in Escherichia coli cells with a disabled phosphorylation cascade. This phosphorylation-independent chemoresponse is explained by reversible inhibition of fumarase by indole or benzoate which leads to an increased level of cellular fumarate, a compound involved in motor switching for bacteria and archaea. Genetic deletion of fumarase increased the intracellular concentration of fumarate and enhanced the switching frequency of the flagellar motors irrespective of the presence or absence of the phosphorylation cascade. These correlations provide evidence for fumaratedependent metabolic signal transduction in bacterial chemosensing.Motile procaryotes actively search for optimal living conditions. This phenomenon, called taxis, is based on the ability to sense environmental stimuli like chemicals, oxygen, and light and to respond by modulating the probability of switching the rotational sense of the motility organelle, the flagellar motor. The interplay of excitation and adaptation allows the cell to detect time-dependent changes in the strengths of different stimulus types; these changes are integrated to give a graded motor response (for reviews, see references 4, 12, and 22). Components of the bacterial sensory machinery have been identified and characterized by genetic, biochemical, and cell physiological methods. Bacterial chemotaxis provides a fascinating example of how computational information processing is performed by a network of proteins (8).

Response regulator output in bacterial chemotaxis

“…In general, however, when comparing results from chemotaxis assays performed under different conditions, it is worth noting that there are factors that can affect motor behavior independently of CheY phosphorylation (Khan and Macnab, 1980;Montrone et al, 1996;Turner et al, 1996;Barak et al, 1998).…”

Chemotactic‐like response of Escherichia coli cells lacking the known chemotaxis machinery but containing overexpressed CheY

“…The response to other attractants was lower, but this could be a result of non-optimal concentrations of the attractants. Interestingly, even benzoate and acetate, which are repellents for wild-type E. coli , increased the probability of CCW rotation at 10 ¹5 M. [At millimolar concentrations, benzoate (Montrone et al, 1996) and acetate (Wolfe et al, 1988) decrease the probability of CCW rotation in Y þ -gutted strains.] The other repellents tested, indole, Ni 2þ and Co 2þ , produced almost no response at 10 ¹5 M but, at the higher concentrations tested, they decreased the probability of CCW rotation.…”

Signal Processing and Flagellar Motor Switching During Phototaxis of Halobacterium salinarum

“…In Halobacterium, it has been shown that the cellular concentration of fumarate is under sensory control of the sensory rhodopsin-transducer complexes, and that there is considerable amplification (in terms of fumarate molecules released per activated sensory rhodopsin; Montrone et al 1993). In E. coli, it was shown (1) that motor switching requires fumarate (Barak and Eisenbach 1992), (2) that the probability of motor switching depends on the cellular fumarate concentration, even in the presence of nonphosphorylatable CheY (Montrone et al 1996(Montrone et al , 1998, (3) that the cellular fumarate level regulates motor switching at low temperatures, even in complete absence of CheY (Prasad et al 1998), and (4) that fumarate acts by lowering the free-energy difference between clockwise and counterclockwise states of the motor (Prasad et al 1998). However, the molecular mechanism of how fumarate interacts with the switch complex and the biochemical mechanism of the sensory control of its cytoplasmic concentration in Halobacterium is unclear at present.…”