Aer and Tsr guide Escherichia coli in spatial gradients of oxidizable substrates

Suzanne E. Greer-Phillips1, Gladys Alexandre2, Barry L. Taylor1 and Igor B. Zhulin1,3

1 Department of Biochemistry and Microbiology, Division of Microbiology and Molecular Genetics, School of Medicine, Loma Linda University, Loma Linda, CA 92350, USA
2 Department of Biology, Georgia State University, Atlanta, GA 30303, USA
3 School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA

Correspondence
Igor B. Zhulin
igor.zhulin{at}biology.gatech.edu


   ABSTRACT
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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The Aer and Tsr chemoreceptors in Escherichia coli govern tactic responses to oxygen and redox potential that are parts of an overall behaviour known as energy taxis. They are also proposed to mediate responses to rapidly utilized carbon sources, glycerol and succinate, via the energy taxis mechanism. In this study, the Aer and Tsr proteins were individually expressed in an ‘all-transducer-knockout’ strain of E. coli and taxis was analysed in gradients of various oxidizable carbon sources. In addition to the known response to glycerol and succinate, it was found that Aer directed taxis towards ribose, galactose, maltose, malate, proline and alanine as well as the phosphotransferase system (PTS) carbohydrates glucose, mannitol, mannose, sorbitol and fructose, but not to aspartate, glutamate, glycine and arabinose. Tsr directed taxis towards sugars (including those transported by the PTS), but not to organic acids or amino acids. When a mutated Aer protein unable to bind the FAD cofactor was expressed in the receptor-less strain, chemotaxis was not restored to any substrate. Aer appears to mediate responses to rapidly oxidizable substrates, whether or not they are effective growth substrates, whereas Tsr appears to mediate taxis to substrates that support maximal growth, whether or not they are rapidly oxidizable. This correlates with the hypothesis that Aer and Tsr sense redox and proton motive force, respectively. Taken together, the results demonstrate that Aer and Tsr mediate responses to a broad range of chemicals and their attractant repertoires overlap with those of specialized chemoreceptors, namely Trg (ribose, galactose) and Tar (maltose).


Abbreviations: ETS, electron transport system; HAMP, histidine kinase, adenylate cyclase, methyl-accepting chemotaxis protein; PAS, period clock protein, aryl hydrocarbon receptor and single-minded protein; PMF, proton motive force; PTS, phosphotransferase system


   INTRODUCTION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Chemotaxis is the ability of motile cells to move along gradients of chemicals. Escherichia coli has a metabolism-independent chemotactic response to various chemicals (e.g. serine, aspartate, maltose, ribose and galactose) that are sensed by specialized transmembrane chemoreceptors via the ligand-binding mechanism (for a review, see Falke & Hazelbauer, 2001). The signals arise directly from binding of a chemical or its complex with a periplasmic protein to the periplasmic domain of one of the four transmembrane chemoreceptors: Tsr (serine), Tar (aspartate, maltose), Trg (ribose, galactose) and Tap (dipeptides). Chemoreceptor signalling controls autophosphorylation of CheA, a cytoplasmic histidine protein kinase that is docked to the receptor via the CheW protein. The phosphoryl moiety is subsequently transferred to the CheY response regulator. Phosphorylated CheY protein binds to the flagellar motor causing a change in flagellar rotation (for reviews, see Parkinson, 1993; Falke et al., 1997). Transmembrane chemoreceptors are highly sensitive: thresholds for Tar and Trg have been established at and below micromolar concentrations of a ligand (Mesibov et al., 1973). In addition, carbohydrates that are transported via the phosphoenolpyruvate-dependent phosphotransferase system (PTS) also cause a metabolism-independent chemotactic response which requires the CheA–CheY pathway and a chemoreceptor(s) (Lux et al., 1999).

E. coli also responds chemotactically to physico-chemical parameters that affect the redox status of the electron transport system (ETS) and proton motive force (PMF). These behavioural responses are collectively known as energy taxis (Taylor & Zhulin, 1998; Taylor et al., 1999; Alexandre & Zhulin, 2001). In energy taxis, cells respond not to a chemical per se, but to changes in the ETS caused by the chemical, i.e. in a metabolism-dependent manner. ‘Stimuli’ for energy taxis include terminal electron acceptors (oxygen, nitrate, etc.) and redox-active chemicals that interfere with electron transport (Taylor et al., 1979; Shioi et al., 1988; Grishanin et al., 1991; Bespalov et al., 1996; Zhulin et al., 1996, 1997; Alexandre et al., 2000).

In energy taxis of E. coli, changes in the ETS are detected by membrane-bound chemoreceptors Aer and Tsr (Bibikov et al., 1997; Rebbapragada et al., 1997). Aer is anchored in the membrane; however, both its sensing and signalling domains are cytoplasmic. It contains a sensory PAS (found in period clock protein, aryl hydrocarbon receptor and single-minded protein) domain (Taylor & Zhulin, 1999) that binds an FAD cofactor potentially capable of changing its redox potential to reflect the status of the ETS (Bibikov et al., 1997, 2000; Rebbapragada et al., 1997; Repik et al., 2000). Because Aer is able to sense and signal changes in the redox potential (Bespalov et al., 1996; Rebbapragada et al., 1997), oxidation and reduction of FAD within the PAS domain is proposed to produce the signal (Taylor et al., 1999; Repik et al., 2000). Aer is not only responsible for taxis to terminal electron acceptors and redox taxis, but was also proposed to mediate positive taxis to glycerol and succinate (Rebbapragada et al., 1997). Both substrates are effective donors of reducing equivalents for the ETS, and it was established in a direct experiment that positive taxis to glycerol in E. coli results from its oxidation (Zhulin et al., 1997). Tsr is a classical ligand-binding transmembrane receptor for serine, but it is also known to govern energy taxis (Rebbapragada et al., 1997). Because Tsr lacks any oxygen- or redox-responsive prosthetic group, it was proposed to sense changes in the PMF by an as yet unknown mechanism. Earlier suggestions that pH-sensing capabilities of Tsr may contribute to PMF sensing (Taylor et al., 1999; Levit & Stock, 1999) are not supported by findings that other transmembrane chemoreceptors in E. coli are also capable of pH-sensing (Yamamoto et al., 1990; Umemura et al., 2002), whereas only Tsr acts as an energy taxis transducer (Rebbapragada et al., 1997). If Aer and Tsr respond to changes in electron transport and/or the PMF, then even in the absence of specialized chemoreceptors, Aer or Tsr should govern cell migration along the gradients of effectively oxidizable substrates.

Here we present experimental evidence that Aer or Tsr when present as a sole chemoreceptor in E. coli mediates metabolism-dependent responses to various oxidizable substrates including some of those that are detected by other chemoreceptors via the ligand-binding metabolism-independent mechanism.


   METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Bacterial strains and growth conditions.
The plasmids and strains used in this study are listed in Table 1. An E. coli mutant (BT3388), derived from the parent strain RP437, lacking all methyl-accepting chemotaxis genes (MCPs) and the aer gene was used in this study (Yu et al., 2002). For growth curve measurements, cells were grown aerobically at 37 °C in minimal media supplemented with the required amino acids and a carbon source to a final concentration of 10 mM. Pre-cultures grown on ribose, galactose, maltose, fructose and/or arabinose were used as inoculum.


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Table 1. Bacterial strains, vectors and plasmids

 
Swarm plate assays for chemotaxis.
Cells were transformed with the indicated plasmids and streaked in a line in semi-soft minimal media plates (0·28 % agar, 100 µg ampicillin ml-1, plus required amino acids). The agar was supplemented with 5 mM succinate and 1–5 mM serine, aspartate, alanine, galactose, ribose, maltose, fructose and/or arabinose for induction depending on the individual substrate to be tested following the selection process. Plates were incubated for 48 h at 30 °C. One to five microlitres of motile cells swimming out from the inoculation line was used to inoculate similar semi-soft plates supplemented with the substrate (10 mM unless otherwise indicated) to be tested and incubated at 30 °C for 20–42 h. The Aer and Tsr proteins are readily expressed from the ‘leaky’ pTrc99a promoter without addition of IPTG (Rebbapragada et al., 1997). To verify the effect of Aer expression levels on chemotaxis, various concentrations of IPTG were added to the plates to induce protein expression from the plasmids. Increasing concentrations of IPTG inhibited the chemotactic response, therefore all experiments were carried out without addition of IPTG. A chemotactic response was observed as the formation of a chemotactic ring and scored on a scale from ‘-’ (no ring) to ‘+++’ (a sharp ring of a large diameter) as described previously (Repik et al., 2000).

Temporal gradient assay for chemotaxis.
Cells were transformed and selected as described above in semi-soft plates, then grown in LB broth at 30 °C and harvested at OD600 0·4. A temporal gradient assay for chemotaxis (Spudich & Koshland, 1975) was performed using a video microscope essentially as described previously (Bespalov et al., 1996). When necessary, the tumbling frequency of individual cells was analysed using a real-time Hobson Bactracker motion analysis system (Hobson Tracking Systems).

Gel electrophoresis and Western analysis.
Cells were grown to OD600 0·4 at 37 °C and induced with 0–200 µM IPTG for 2 h. Cells were then centrifuged and resuspended in SDS sample buffer (2 % SDS, 1 % 2-mercaptoethanol, 15 % glycerol and 62 mM Tris base, pH 6·8), boiled for 2 min and applied to SDS-10 % polyacrylamide gels. The resolved proteins were electroblotted to nitrocellulose membranes and probed. Aer was detected using an anti-N-terminal Aer polyclonal antibody (a gift from M. S. Johnson, Loma Linda University School of Medicine, CA, USA) and Tsr was detected using an anti-Tsr polyclonal antibody (a gift from J. S. Parkinson, University of Utah, Salt Lake City, UT, USA). Expression of both Aer and Tsr from the plasmids was detected even without addition of IPTG, whereas due to the low copy numbers no Aer was detected in wild-type cells.


   RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Aer mediates taxis to various oxidizable substrates
An all-transducer-knockout strain, BT3388 (Yu et al., 2002), was transformed with a plasmid harbouring the aer gene under control of the trc lactose inducible promoter, pGH1 (Rebbapragada et al., 1997), or pAVR2 (Repik et al., 2000). Chemotaxis of transformed cells was assayed in minimal media swarm plates containing the substrate to be tested as a chemoeffector at a concentration of 10 mM. The substrates tested and the relative chemotactic response observed are listed in Table 2. Selected results are shown in Fig. 1 to illustrate the scoring of the chemotactic response. We found that Aer, as the only chemoreceptor in this strain, was able to direct bacterial taxis in gradients of oxidizable organic acids, sugars including the PTS substrates, and amino acids. Aer-expressing cells formed a sharp chemotactic ring typical of a strong chemotactic response in a spatial gradient of a stimulus, whereas the receptor-less strain carrying the vector alone grew but did not produce a chemotactic ring on any substrate tested (Fig. 1). Chemotaxis to galactose, maltose and ribose was observed only in cells grown on these substrates prior to testing.


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Table 2. Comparison of chemotactic responses to selected carbon sources in E. coli strains expressing only the Aer or Tsr protein

Chemotaxis was determined as described in Methods and shown in Fig. 1. Cells were grown as described in Methods in swarm plates containing 10 mM substrate to be tested as a sole carbon and energy source. The mutant strain BT3388(pAVR26) (Aer-) did not show chemotaxis towards any of the substrates tested.

 


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Fig. 1. Aer alone governs E. coli taxis in spatial gradients of oxidizable substrates. (1) BT3388(pTrc99A) (all-transducer-knockout strain carrying a vector); (2–5) BT3388(pGH1) (the same strain expressing aer): (2) arabinose, 10 mM; (3) glycerol, 10 mM; (4) fructose, 10 mM; (5) malate, 10 mM. Similar results were obtained with the strain expressing Aer from the pAVR2 plasmid. The chemotactic response was scored on a scale from ‘+++’ to ‘-’ according to the presence and diameter of the chemotactic ring, as described previously (Repik et al., 2000). Notice the complete absence of the chemotactic ring in a strain lacking all chemotaxis transducers. The experimental procedure is described in Methods.

 
The BT3388 cells had a smooth swimming bias even when expressing the Aer protein. This made it difficult to measure positive (smooth swimming) chemotactic responses to oxidizable substrates in a temporal gradient assay (insufficient number of tumblings per cell per run to be detected by the motion analysis system). However, the Aer-expressing cells showed a strong negative response (>90 % tumbling bias for >4 min) upon addition of 2 µM redox repellent 2,4-benzoquinone, which triggers a redox tactic response (Bespalov et al., 1996), i.e. were responding to temporal stimulation similarly to wild-type cells. This response was absent in the all-transducer-knockout strain.

Redox-responsive co-factor of Aer is required for taxis to oxidizable substrates
To verify whether Aer-governed motility along gradients of oxidizable substrates was due to its redox-sensing capabilities (i.e. occurred via the energy taxis mechanism), we tested an Aer mutant [BT3388(pAVR26), aer D68C] unable to bind the redox-responsive FAD cofactor (Repik et al., 2000). The results of these experiments are shown in Table 2. Behavioural assays were carried out on semi-soft swarm plates supplemented with IPTG to induce expression of the mutant Aer protein, if necessary. Protein expression was verified by SDS-PAGE (data not shown). The mutant was unable to show chemotaxis to any substrate tested over a range of IPTG concentrations from 0 to 200 µM. The mutant also lost the negative response to 2,4-benzoquinone in a temporal gradient assay.

Tsr governs chemotaxis in gradients of effective growth substrates
Tsr was expressed in BT3388 by transforming the cells with a plasmid (pJL3) harbouring the tsr gene under control of the trc lactose inducible promoter (Rebbapragada et al., 1997) and chemotaxis was assayed similarly to that for strains expressing Aer. Protein expression was verified by Western blotting using anti-Tsr antiserum supplied by J. S. Parkinson. We found that Tsr-carrying cells formed chemotactic rings in gradients of the sugars ribose, galactose, maltose, glucose, fructose, mannose, sorbitol and glycerol, and to a significantly lesser extent in gradients of proline and alanine. No chemotactic rings were formed by the Tsr-carrying strain in gradients of succinate, malate, glutamate and aspartate (Table 2).

Tsr preferentially directed taxis in gradients of sugars that are the most efficient growth substrates for E. coli. To evaluate further this apparent relationship between energy efficiency and chemotaxis, we measured doubling times of the wild-type cells grown on different substrates in minimal medium. We found a correlation between the efficiency of the chemical as a stimulus for Tsr-mediated chemotaxis and as a growth substrate. The best chemoeffectors for Tsr, such as fructose, glucose and maltose, were also the best growth substrates. Chemicals that did not elicit a chemotactic response, such as succinate and arabinose, were poor or were not carbon substrates for growth (Table 3). For the Aer-mediated response, no correlation was found between the efficiency of a chemical as an attractant and a growth substrate.


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Table 3. Correlation between the efficiency of chemicals as carbon sources and attractants in the Tsr-governed chemotactic response

Cells were grown as described in Methods.

 
Aer and Tsr have a high threshold for responses to oxidizable substrates
Aer and Tsr as sole chemoreceptors in the all-transducer-knockout strain BT3388 were able to guide cells in gradients of maltose, ribose and galactose, chemicals previously thought to be recognized only by the dedicated ligand-binding chemoreceptors Tar and Trg. If Aer and Tsr are capable of directing taxis to ribose and galactose, then the trg mutant of E. coli should have a chemotactic response to these chemicals due to the presence of Aer and Tsr. The reason that this response in the trg mutant has never been reported might be due to the higher concentration of ribose and galactose required for the Aer- and Tsr-mediated metabolism-dependent responses in comparison with the Trg-mediated metabolism-independent response. To test this hypothesis, we established the threshold for chemotaxis to ribose and galactose in the wild-type E. coli (RP437) and a strain lacking the Trg receptor (CP177). In swarm plates supplemented with decreasing concentrations of ribose or galactose (10 mM to 10 µM), we found that the wild-type (Trg-mediated response) was able to produce chemotactic rings on swarm plates at a 10-5 M concentration of ribose and galactose, whereas the {Delta}trg strain (Aer- and Tsr-mediated response) was unable to form chemotactic rings at concentrations below 10-3 M ribose or galactose (Fig. 2). Temporal gradient assays produced similar results, where a 2–3 orders of magnitude difference in the threshold was observed between the two strains (data not shown).



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Fig. 2. Chemotactic responses of wild-type E. coli and a trg strain to decreasing concentrations of ribose on swarm plates. Top, RP437 (wild-type); bottom, CP177 (trg). Ribose concentration: (1), 10 mM; (2), 1 mM; (3), 100 µM; (4), 10 µM. The experimental procedure is described in Methods.

 
In another experiment, we compared the sensing threshold of Aer and Tar for the chemoeffector maltose, which is detected by Tar via a classical metabolism-independent (ligand-binding) mechanism. BT3388 expressing Aer (pGH1) was compared to a strain lacking both Tsr and Aer (BT3312) in swarm plates supplemented with decreasing concentrations of maltose (10 mM to 10 µM). In BT3388(pGH1) (aer+), the chemotactic response to maltose would be limited to energy taxis by Aer, and in BT3312 (aer tsr), chemotaxis would be limited to the ligand-binding recognition by Tar. We found BT3312 to be chemotactic towards maltose in swarm plates at 10-5 M. However, BT3388(pGH1) (aer+) showed no chemotaxis to maltose below 10-3 M (data not shown).


   DISCUSSION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Energy taxis is an efficient way for cells to monitor their environment. Aer has been shown to act as a transducer for energy taxis (Rebbapragada et al., 1997; Repik et al., 2000). Studies with aer and tsr mutants suggested that the Aer and Tsr proteins in E. coli direct taxis in spatial gradients of succinate and glycerol (Rebbapragada et al., 1997). An all-transducer-knockout strain of E. coli provided a unique opportunity to test Aer and Tsr individually for their ability to direct cell motility along the gradients of various chemical substrates. We demonstrated that both Aer and Tsr were functional when present as the sole chemoreceptor in a cell. Aer was able to mediate taxis towards organic acids, sugars and some amino acids in a spatial chemical gradient. We found that succinate caused the strongest behavioural response by the Aer-carrying cells. The response to succinate and to all other oxidizable substrates was dependent on the presence of the FAD cofactor in Aer, which is consistent with the proposed sensing mechanism. Succinate dehydrogenase is an FAD-containing flavoprotein and an immediate entry point into the ETS of E. coli; therefore, succinate oxidation immediately affects the redox status of the ETS component(s), which interact with FAD. This may explain why succinate causes the strongest behavioural response by the FAD-containing Aer transducer. The mechanism of redox signalling by the Aer-associated flavin has been proposed recently (Taylor et al., 1999), where the semiquinone form signals as an attractant, while both oxidized and reduced forms signal as repellents.

We found that Tsr efficiently governs chemotaxis in gradients of sugars, which are the best growth substrates for E. coli, and demonstrated a correlation between the efficiency of a chemical as a growth substrate and an attractant. Such a correlation is indicative of energy taxis (Alexandre et al., 2000). The substrates that caused strong responses for Aer- and Tsr-mediated behaviours were different. This difference is consistent with the hypothesis that Aer and Tsr mediate energy taxis by different mechanisms, redox and PMF sensing, respectively (Taylor et al., 1999). If PMF sensing is indeed a mechanism by which Tsr governs energy taxis, then it remains to be seen why other chemotaxis transducers that are sensitive to internal pH (Yamamoto et al., 1990; Umemura et al., 2002) are not energy taxis sensors. One of the possible explanations is that different amino acid residues of transducers are involved in pH taxis versus energy taxis signalling. For example, the residues that are involved in pH sensing were found in the cytoplasmic HAMP domain (Umemura et al., 2002), whereas residues that might control the transducer mobility in the membrane in response to changes in the electrochemical proton gradient are likely to be located in the membrane/cytoplasm interface (i.e. between the transmembrane helix and the HAMP domain), as suggested for other transmembrane proteins (Schuenemann et al., 1999).

The energy taxis receptors Aer and Tsr were found to be insensitive to substrates at low concentrations compared to the specialized chemotaxis receptors Tar and Trg. Aer and Tsr are capable of navigating E. coli cells in gradients of ribose, galactose and maltose, but only at concentrations above 1 mM, whereas Tar and Trg sense these sugars at 10 µM concentrations (in a spatial gradient assay). This low sensitivity is typical of the energy taxis responses (Zhulin et al., 1997; Alexandre et al., 2000) and likely reflects the necessity for a high substrate concentration in order to support electron flow via the ETS and generation of the PMF.

Taken together, our results demonstrate that energy taxis receptors of E. coli even in the absence of highly sensitive specialized chemoreceptors enable the cells to navigate towards a preferred concentration of carbon and energy sources. Energy taxis is a dominant type of behaviour in some species (Alexandre et al., 2000) and is proposed to be widespread in bacteria (Taylor et al., 1999; Alexandre & Zhulin, 2001). By widening the repertoire of chemicals that are detected by energy taxis receptors, this study further supports this hypothesis.


   ACKNOWLEDGEMENTS
 
We thank Gerald Hazelbauer for the gift of strain CP177, Sandy Parkinson for the gift of anti-Tsr antibody and Kylie Watts for providing strain BT3388 prior to publication. We also thank Mark Johnson for the anti-Aer antibody, expert advice and critical reading of the manuscript.

This work was supported, in part, by the National Institutes of Health grant GM29481 (to B. L. T.) and by the start-up funds from Georgia State University (to G. A.) and Georgia Institute of Technology (to I. B. Z.).


   REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Alexandre, G. & Zhulin, I. B. (2001). More than one way to sense chemicals. J Bacteriol 183, 4681–4686.[Free Full Text]

Alexandre, G., Greer, S. E. & Zhulin, I. B. (2000). Energy taxis is the dominant behavior in Azospirillum brasilense. J Bacteriol 182, 6042–6048.[Abstract/Free Full Text]

Bespalov, V. A., Zhulin, I. B. & Taylor, B. L. (1996). Behavioral responses of Escherichia coli to changes in redox potential. Proc Natl Acad Sci U S A 93, 10084–10089.[Abstract/Free Full Text]

Bibikov, S. I., Biran, R., Rudd, K. E. & Parkinson, J. S. (1997). A signal transducer for aerotaxis in Escherichia coli. J Bacteriol 179, 4075–4079.[Abstract]

Bibikov, S. I., Barnes, L. A., Gitin, Y. & Parkinson, J. S. (2000). Domain organization and flavin adenine dinucleotide-binding determinants in the aerotaxis signal transducer Aer of Escherichia coli. Proc Natl Acad Sci U S A 97, 5830–5835.[Abstract/Free Full Text]

Falke, J. J. & Hazelbauer, G. L. (2001). Transmembrane signaling in bacterial chemoreceptors. Trends Biochem Sci 26, 257–265.[CrossRef][Medline]

Falke, J. J., Bass, R. B., Butler, S. L., Chervitz, S. A. & Danielson, M. A. (1997). The two-component signaling pathway of bacterial chemotaxis: a molecular view of signal transduction by receptors, kinases, and adaptation enzymes. Annu Rev Cell Dev Biol 13, 457–512.[CrossRef][Medline]

Grishanin, R. N., Chalmina, I. I. & Zhulin, I. B. (1991). Behaviour of Azospirillum brasilense in a spatial gradient of oxygen and in a ‘redox’ gradient of an artificial electron acceptor. J Gen Microbiol 137, 2781–2785.

Levit, M. N. & Stock, J. B. (1999). pH sensing in bacterial chemotaxis. Novartis Found Symp 221, 38–50.[Medline]

Lux, R., Munasinghe, V. R., Castellano, F., Lengeler, J. W., Corrie, J. E. & Khan, S. (1999). Elucidation of a PTS-carbohydrate chemotactic signal pathway in Escherichia coli using a time-resolved behavioral assay. Mol Biol Cell 10, 1133–1146.[Abstract/Free Full Text]

Mesibov, R., Ordal, G. W. & Adler, J. (1973). The range of attractant concentrations for bacterial chemotaxis and the threshold and size of response over this range. Weber law and related phenomena. J Gen Physiol 62, 203–223.[Abstract/Free Full Text]

Parkinson, J. S. (1993). Signal transduction schemes of bacteria. Cell 73, 857–871.[Medline]

Parkinson, J. S. & Houts, S. E. (1982). Isolation and behavior of Escherichia coli deletion mutants lacking chemotaxis functions. J Bacteriol 151, 106–113.[Medline]

Rebbapragada, A., Johnson, M. S., Harding, G. P., Zuccarelli, A. J., Fletcher, H. M., Zhulin, I. B. & Taylor, B. L. (1997). The Aer protein and the serine chemoreceptor Tsr independently sense intracellular energy levels and transduce oxygen, redox, and energy signals for Escherichia coli behavior. Proc Natl Acad Sci U S A 94, 10541–10546.[Abstract/Free Full Text]

Repik, A., Rebbapragada, A., Johnson, M. S., Haznedar, J. O., Zhulin, I. B. & Taylor, B. L. (2000). PAS domain residues involved in signal transduction by the Aer redox sensor of Escherichia coli. Mol Microbiol 36, 806–816.[CrossRef][Medline]

Schuenemann, T. A., Delgado-Nixon, V. M. & Dalbey, R. E. (1999). Direct evidence that the proton motive force inhibits membrane translocation of positively charged residues within membrane proteins. J Biol Chem 274, 6855–6864.[Abstract/Free Full Text]

Shioi, J., Tribhuwan, R. C., Berg, S. T. & Taylor, B. L. (1988). Signal transduction in chemotaxis to oxygen in Escherichia coli and Salmonella typhimurium. J Bacteriol 170, 5507–5511.[Medline]

Spudich, J. L. & Koshland, D. E. J. (1975). Quantitation of the sensory response in bacterial chemotaxis. Proc Natl Acad Sci U S A 72, 710–713.[Abstract]

Taylor, B. L. & Zhulin, I. B. (1998). In search of higher energy: metabolism-dependent behaviour in bacteria. Mol Microbiol 28, 683–690.[CrossRef][Medline]

Taylor, B. L. & Zhulin, I. B. (1999). PAS domains: internal sensors of oxygen, redox potential and light. Microbiol Mol Biol Rev 63, 479–506.[Abstract/Free Full Text]

Taylor, B. L., Miller, J. B., Warrick, H. M. & Koshland, D. E., Jr (1979). Electron acceptor taxis and blue light effect on bacterial chemotaxis. J Bacteriol 140, 567–573.[Medline]

Taylor, B. L., Zhulin, I. B. & Johnson, M. S. (1999). Aerotaxis and other energy-sensing behavior in bacteria. Annu Rev Microbiol 53, 103–128.[CrossRef][Medline]

Umemura, T., Matsumoto, Y., Ohnishi, K., Homma, M. & Kawagishi, I. (2002). Sensing of cytoplasmic pH by bacterial chemoreceptors involves the linker region that connects the membrane-spanning and the signal-modulating helices. J Biol Chem 277, 1593–1598.[Abstract/Free Full Text]

Yamamoto, K., Macnab, R. M. & Imae, Y. (1990). Repellent response functions of the Trg and Tap chemoreceptors of Escherichia coli. J Bacteriol 172, 383–388.[Medline]

Yu, H. S., Saw, J. H., Hou, S. & 7 other authors (2002). Aerotactic responses in bacteria to photoreleased oxygen. FEMS Microbiol Lett 217, 237–242.[CrossRef][Medline]

Zhulin, I. B., Bespalov, V. A., Johnson, M. S. & Taylor, B. L. (1996). Oxygen taxis and proton motive force in Azospirillum brasilense. J Bacteriol 178, 5199–5204.[Abstract]

Zhulin, I. B., Rowsell, E. H., Johnson, M. S. & Taylor, B. L. (1997). Glycerol elicits energy taxis of Escherichia coli and Salmonella typhimurium. J Bacteriol 179, 3196–3201.[Abstract]

Received 17 February 2003; revised 2 May 2003; accepted 5 May 2003.