Scholarly Journals--Published

  • New Roles for HAMP Domains: the Tri-HAMP Region of Pseudomonas aeruginosa Aer2 Controls Receptor Signaling and Cellular Localization   Selina Anaya, Emilie Orillard, Suzanne E. Greer-Phillips, Kylie J. Watts DOI: The Aer2 chemoreceptor from Pseudomonas aeruginosa is an O2 sensor involved in stress responses, virulence, and tuning the behavior of the chemotaxis (Che) system. Aer2 is the sole receptor of the Che2 system. It is soluble, but membrane associated, and forms complexes at the cell pole during stationary phase. The domain arrangement of Aer2 is unusual, with a PAS sensing domain sandwiched between five HAMP domains, followed by a C-terminal kinase-control output domain. The first three HAMP domains form a poly-HAMP chain N-terminal to the PAS sensing domain. HAMP domains are often located between signal input and output domains, where they transduce signals. Given that HAMP1 to 3 reside N-terminal to the input-output pathway, we undertook a systematic examination of their function in Aer2. We found that HAMP1 to 3 influence PAS signaling over a considerable distance, as the majority of HAMP1, 2 and 3 mutations, and deletions of helical phase stutters, led to nonresponsive signal-off or off-biased receptors. PAS signal-on lesions that mimic activated Aer2 also failed to override N-terminal HAMP signal-off replacements. This indicates that HAMP1 to 3 are critical coupling partners for PAS signaling and likely function as a cohesive unit and moveable scaffold to correctly orient and poise PAS dimers for O2-mediated signaling in Aer2. HAMP1 additionally controlled the clustering and polar localization of Aer2 in P. aeruginosa. Localization was not driven by HAMP1 charge, and HAMP1 signal-off mutants still localized. Employing HAMP as a clustering and localization determinant, as well as a facilitator of PAS signaling, are newly recognized roles for HAMP domains. (08/2022)
  • "THE AER2 RECEPTOR FROM VIBRIO CHOLERAE IS A DUAL PAS-HEME OXYGEN SENSOR" Greer-Phillips SE, Sukomon N, Chua TK, Johnson MS, Crane BR, Watts K. Mol Microbiol. 2018 May 2. doi: 10.1111/mmi.13978. [Epub ahead of print] (05/2018) (link)
  • The Response Regulator RRG-1 Functions Upstream of a Mitogen-activated Protein Kinase Pathway Impacting Asexual Development, Female Fertility, Osmotic Stress, and Fungicide Resistance in Neurospora crassa Carol A. Jones,Suzanne E. Greer-Phillip, Katherine A. Borkovich   28 Mar 2007 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 Δrrg-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, Δrrg-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, Δrrg-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 Δrrg-1 strains. OS-2-phosphate levels are also low in Δrrg-1, os-2, and os-4 mutants under nitrogen starvation. Analysis of the rrg-1D921N 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. (03/2007) (link)
  • An Energy Taxis Transducer Promotes Root Colonization by Azospirillum brasilense   Suzanne E. Greer-Phillips, Bonnie B. Stephens, Gladys Alexandre DOI: 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. (10/2004)
  • Ecological role of energy taxis in microorganisms Gladys Alexandre, Suzanne Greer-Phillips, Igor B Zhulin FEMS microbiology reviews Volume 28, Issue 1, February 2004, Pages 113–126 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. (02/2004) (link)
  • Energy Taxis Is the Dominant Behavior in Azospirillum brasilense   Gladys Alexandre, Suzanne E. Greer, Igor B. Zhulin DOI: Energy taxis encompasses aerotaxis, phototaxis, redox taxis, taxis to alternative electron acceptors, and chemotaxis to oxidizable substrates. The signal for this type of behavior is originated within the electron transport system. Energy taxis was demonstrated, as a part of an overall behavior, in several microbial species, but it did not appear as the dominant determinant in any of them. In this study, we show that most behavioral responses proceed through this mechanism in the alpha-proteobacterium Azospirillum brasilense. First, chemotaxis to most chemoeffectors typical of the azospirilla habitat was found to be metabolism dependent and required a functional electron transport system. Second, other energy-related responses, such as aerotaxis, redox taxis, and taxis to alternative electron acceptors, were found in A. brasilense. Finally, a mutant lacking a cytochromec oxidase of the cbb3 type was affected in chemotaxis, redox taxis, and aerotaxis. Altogether, the results indicate that behavioral responses to most stimuli inA. brasilense are triggered by changes in the electron transport system. (11/2000)