Motile microorganisms rapidly respond to changes in various physico-chemical gradients by directing their motility to more favorable surroundings. Energy generation is one of the most important parameters for the survival of microorganisms in their environment. Therefore it is not surprising that microorganisms are able to monitor changes in the cellular energy generating processes. The signal for this behavioral response, which is called energy taxis, originates within the electron transport system. By coupling energy metabolism and behavior, energy taxis is fine-tuned to the environment a cell finds itself in and allows efficient adaptation to changing conditions that affect cellular energy levels. Thus, energy taxis provides cells with a versatile sensory system that enables them to navigate to niches where energy generation is optimized. This behavior is likely to govern vertical species stratification and the active migration of motile cells in response to shifting gradients of electron donors and/or acceptors which are observed within microbial mats, sediments and soil pores. Energy taxis has been characterized in several species and might be widespread in the microbial world. Genome sequencing revealed that many microorganisms from aquatic and soil environments possess large numbers of chemoreceptors and are likely to be capable of energy taxis. In contrast, species that have a fewer number of chemoreceptors are often found in specific, confined environments, where relatively constant environmental conditions are expected. Future studies focusing on characterizing behavioral responses in species that are adapted to diverse environmental conditions should unravel the molecular mechanisms underlying sensory behavior in general and energy taxis in particular. Such knowledge is critical to a better understanding of the ecological role of energy taxis.

Two-component systems, consisting of proteins with histidine kinase and/or response regulator domains, regulate environmental responses in bacteria, Archaea, fungi, slime molds, and plants. Here, we characterize RRG-1, a response regulator protein from the filamentous fungus Neurospora crassa. The cell lysis phenotype of ⌬rrg-1 mutants is reminiscent of osmotic-sensitive (os) mutants, including nik-1/os-1 (a histidine kinase) and strains defective in components of a mitogen-activated protein kinase (MAPK) pathway: os-4 (MAPK kinase kinase), os-5 (MAPK kinase), and os-2 (MAPK). Similar to os mutants, ⌬rrg-1 strains are sensitive to hyperosmotic conditions, and they are resistant to the fungicides fludioxonil and iprodione. Like os-5, os-4, and os-2 mutants, but in contrast to nik-1/os-1 strains, ⌬rrg-1 mutants do not produce female reproductive structures (protoperithecia) when nitrogen starved. OS-2-phosphate levels are elevated in wild-type cells exposed to NaCl or fludioxonil, but they are nearly undetectable in ⌬rrg-1 strains. OS-2-phosphate levels are also low in ⌬rrg-1, os-2, and os-4 mutants under nitrogen starvation. Analysis of the rrg-1 D921N allele, mutated in the predicted phosphorylation site, provides support for phosphorylation-dependent and -independent functions for RRG-1. The data indicate that RRG-1 controls vegetative cell integrity, hyperosmotic sensitivity, fungicide resistance, and protoperithecial development through regulation of the OS-4/OS-5/OS-2 MAPK pathway. INTRODUCTIONTwo-component regulatory systems are signal transduction pathways found in bacteria, Archaea, slime molds, fungi, and plants (for reviews, see Wolanin et al., 2002). Twocomponent systems have been implicated in responses to light, osmolarity, cellular redox status, nutrient and oxygen levels, virulence, and other factors (for review, see Wolanin et al., 2002;Borkovich et al., 2004). A basic two-component system consists of a histidine kinase and a response regulator (for review, see West and Stock, 2001;Wolanin et al., 2002). Histidine kinases autophosphorylate on a conserved histidine residue in the histidine kinase domain. The phosphoryl group is then transferred to an aspartate residue in the receiver domain on the response regulator. Complex two-component signaling pathways (also termed phosphorelays) contain hybrid histidine kinases that have both a histidine kinase domain and a response regulator receiver domain in the same protein, thus facilitating an intramolecular phosphate transfer reaction. The phosphate group on the aspartate residue is transferred to a histidine phosphotransfer (HPT) protein. Subsequently, the histidine phosphotransfer protein phosphorylates the receiver domain on a response regulator. Phosphorelays are the major two-component pathways in eukaryotic systems; however, two-component systems are not present in animals, making them ideal candidate targets for antimicrobial drugs (for review, see Alex et al., 1998;Wolanin et al., 2002;Borkovich et al., 2004).Many response regulators contain a ...

Motility responses triggered by changes in the electron transport system are collectively known as energy taxis. In Azospirillum brasilense, energy taxis was shown to be the principal form of locomotor control. In the present study, we have identified a novel chemoreceptor-like protein, named Tlp1, which serves as an energy taxis transducer. The Tlp1 protein is predicted to have an N-terminal periplasmic region and a cytoplasmic C-terminal signaling module homologous to those of other chemoreceptors. The predicted periplasmic region of Tlp1 comprises a conserved domain that is found in two types of microbial sensory receptors: chemotaxis transducers and histidine kinases. However, the function of this domain is currently unknown. We characterized the behavior of a tlp1 mutant by a series of spatial and temporal gradient assays. The tlp1 mutant is deficient in (i) chemotaxis to several rapidly oxidizable substrates, (ii) taxis to terminal electron acceptors (oxygen and nitrate), and (iii) redox taxis. Taken together, the data strongly suggest that Tlp1 mediates energy taxis in A. brasilense. Using qualitative and quantitative assays, we have also demonstrated that the tlp1 mutant is impaired in colonization of plant roots. This finding supports the hypothesis that energy taxis and therefore bacterial metabolism might be key factors in determining host specificity in Azospirillum-grass associations.Azospirillum brasilense, a free-living diazotroph that belongs to the alpha-subdivision of proteobacteria, associates with the roots of many agriculturally important crops, including wheat, corn, and rice. Azospirilla colonize the root surface and may significantly promote plant growth and crop yield, properties that make them attractive candidates for the development of biological fertilizers for these crops (26-28). The ability of Azospirillum to attain significant populations on the root surfaces of the host is essential for its beneficial effect on plant growth and requires that the bacteria come in close contact with the roots (9,27,28,35). The abilities of sensing chemicals released by the host plant and navigating toward the root system are likely to be important for the establishment of bacteria, including Azospirillum (9, 10, 42), in the rhizosphere. Experimental evidence supporting this hypothesis was obtained by demonstrating that nonchemotactic and nonmotile mutants of A. brasilense are severely impaired in surface colonization of wheat roots (41). Although there is no strict host specificity in Azospirillum-plant associations, a strain-specific chemotaxis was reported: strains isolated from the rhizosphere of a particular grass demonstrated preferential chemotaxis toward chemicals found in root exudates of that grass (31). These results suggested that chemotaxis may contribute to host-plant specificity and could largely be determined by metabolism (31).Most motility responses in A. brasilense are a particular form of metabolism-dependent taxis called energy taxis (1). In energy taxis, a flow of reducing equ...

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 'alltransducer-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).

The diarrheal pathogen Vibrio cholerae navigates complex environments using three chemosensory systems and 44-45 chemoreceptors. Chemosensory cluster II modulates chemotaxis, whereas clusters I and III have unknown functions. Ligands have been identified for only five V. cholerae chemoreceptors. Here we report that the cluster III receptor, VcAer2, binds and responds to O . VcAer2 is an ortholog of Pseudomonas aeruginosa Aer2 (PaAer2), but differs in that VcAer2 has two, rather than one, N-terminal PAS domain. We have determined that both PAS1 and PAS2 form homodimers and bind penta-coordinate b-type heme via an Eη-His residue. Heme binding to PAS1 required the entire PAS core, but receptor function also required the N-terminal cap. PAS2 functioned as an O -sensor [K , 19 μM], utilizing the same Iβ Trp (W276) as PaAer2 to stabilize O . The crystal structure of PAS2-W276L was similar to that of PaAer2-PAS, but resided in an active conformation mimicking the ligand-bound state, consistent with its signal-on phenotype. PAS1 also bound O [K 12 μM], although O binding was stabilized by either a Trp or Tyr residue. Moreover, PAS1 appeared to function as a signal modulator, regulating O -mediated signaling from PAS2, and resulting in activation of the cluster III chemosensory pathway. This article is protected by copyright. All rights reserved.