The Aer protein and the serine chemoreceptor Tsr independently sense intracellular energy levels and transduce oxygen, redox, and energy signals for Escherichia coli behavior

Abstract

We identified a protein, Aer, as a signal transducer that senses intracellular energy levels rather than the external environment and that transduces signals for aerotaxis (taxis to oxygen) and other energy-dependent behavioral responses in Escherichia coli. Domains in Aer are similar to the signaling domain in chemotaxis receptors and the putative oxygen-sensing domain of some transcriptional activators. A putative FAD-binding site in the N-terminal domain of Aer shares a consensus sequence with the NifL, Bat, and Wc-1 signal-transducing proteins that regulate gene expression in response to redox changes, oxygen, and blue light, respectively. A double mutant deficient in aer and tsr, which codes for the serine chemoreceptor, was negative for aerotaxis, redox taxis, and glycerol taxis, each of which requires the proton motive force and/or electron transport system for signaling. We propose that Aer and Tsr sense the proton motive force or cellular redox state and thereby integrate diverse signals that guide E. coli to environments where maximal energy is available for growth.

Figure 1

The domain structure of the Aer protein in E. coli and comparison of its deduced amino acid sequence to homologous proteins. Similar residues are highlighted; identical residues are in bold. (A) Multiple alignment of the N-terminal domain of Aer with homologous domains of three related proteins, NifL from A. vinelandii, Bat from H. salinarium, and Wc-1 from N. crassa. (B) Alignment of the transmembrane region of Aer with the transmembrane regions (TM1 and TM2) of Tsr. Three positively charged residues (+) and the amphipathic sequence (AS) adjacent to the transmembrane regions of the Tsr protein are indicated. (C) Alignment of the C-terminal domains of Aer and Tsr. Methylation regions, K1 and R1, and the highly conserved domain (HCD) are shown. The Glx–Glx doublets corresponding to characterized methylation sites of Tsr (37) are marked with an asterisk. See text for details.

Figure 2

Comparison of aerotactic behavioral responses in wild-type and mutant strains of E. coli. (A) Aerotaxis of E. coli RP437 and isogenic mutants, BT3309 (aer), RP5882 (tsr), and BT3311 (aer, tsr) in a spatial oxygen gradient. A capillary assay was performed as described in Materials and Methods. Photographs were taken 10 min after the cells were placed into capillaries. The meniscus at the air interface is visible at the right in the photographs. (B) Aerotactic response in an aer mutant (BT3300) and its parent (MM335) in a temporal gradient assay using computerized motion analysis as described in Materials and Methods. Arrows indicate addition (↓) and removal (↑) of oxygen. The MM335 strain shows enhanced aerotaxis and chemotaxis responses compared with RP437.

Figure 3

The response of E. coli to an oxygen decrease as a function of expression of aer. Wild-type MM335 cells transformed with pGH1 were grown to mid-log phase (OD₆₀₀ = 0.4–0.5) and induced with the indicated concentrations of IPTG for 1 h before analysis of aerotaxis by a temporal assay. In the presence of 1 mM IPTG, the cells did not adapt in the 600 s that they were observed.

Figure 4

Model for energy-sensing by Aer and Tsr. Aer senses modulators of the electron transport system. This is postulated to be mediated by the FAD cofactor of Aer. Tsr may sense the proton motive force directly or indirectly through changes in the electron transport system. Homologous signaling domains in Aer and Tsr bind to the CheA/CheW complex and ultimately regulate the level of phosphorylation of the CheY response regulator. A, CheA sensor kinase; W, CheW docking protein; Y, CheY response regulator; Z, CheZ phosphatase.