The Only Chemoreceptor Encoded by che Operon Affects the Chemotactic Response of Agrobacterium to Various Chemoeffectors

Chemoreceptor (also called methyl-accepting chemotaxis protein, MCP) is the leading signal protein in the chemotaxis signaling pathway. MCP senses and binds chemoeffectors, specifically, and transmits the sensed signal to downstream proteins of the chemotaxis signaling system. The genome of Agrobacterium fabrum (previously, tumefaciens) C58 predicts that a total of 20 genes can encode MCP, but only the MCP-encoding gene atu0514 is located inside the che operon. Hence, the identification of the exact function of atu0514-encoding chemoreceptor (here, named as MCP514) will be very important for us to understand more deeply the chemotaxis signal transduction mechanism of A. fabrum. The deletion of atu0514 significantly decreased the chemotactic migration of A. fabrum in a swim plate. The test of atu0514-deletion mutant (Δ514) chemotaxis toward single chemicals showed that the deficiency of MCP514 significantly weakened the chemotactic response of A. fabrum to four various chemicals, sucrose, valine, citric acid and acetosyringone (AS), but did not completely abolish the chemotactic response. MCP514 was localized at cell poles although it lacks a transmembrane (TM) region and is predicted to be a cytoplasmic chemoreceptor. The replacement of residue Phe328 showed that the helical structure in the hairpin subdomain of MCP514 is a direct determinant for the cellular localization of MCP514. Single respective replacements of key residues indicated that residues Asn336 and Val353 play a key role in maintaining the chemotactic function of MCP514.

Does the Rainbow Trout Ovarian Fluid Promote the Spermatozoon on Its Way to the Egg?

The fertilization of freshwater fish occurs in an environment that may negatively affect the gametes; therefore, the specific mechanisms triggering the encounters of gametes would be highly expedient. The egg and ovarian fluid are likely the major sources of these triggers, which we confirmed here for rainbow trout (Oncorhynchus mykiss). The ovarian fluid affected significantly spermatozoa performance: it supported high velocity for a longer period and changed the motility pattern from tumbling in water to straightforward moving in the ovarian fluid. Rainbow trout ovarian fluid induced a trapping chemotaxis-like effect on activated male gametes, and this effect depended on the properties of the activating medium. The interaction of the spermatozoa with the attracting agents was accompanied by the “turn-and-run” behavior involving asymmetric flagellar beating and Ca2+ concentration bursts in the bent flagellum segment, which are characteristic of the chemotactic response. Ovarian fluid created the optimal environment for rainbow trout spermatozoa performance, and the individual peculiarities of the egg (ovarian fluid)–sperm interaction reflect the specific features of the spawning process in this species.

Does the rainbow trout ovarian fluid guide the spermatozoon on its way to the egg?

Fertilization of freshwater fish occurs in an environment that may affect negatively the gametes, therefore the specific mechanisms triggering the encounters of gametes would be highly expedient. The egg and ovarian fluid (OF) are likely the major sources of these triggers in fish, that we confirmed here for rainbow trout Oncorhynchus mykiss. The ovarian fluid affected significantly the spermatozoa performance: it supported high velocity for a longer period and changed the motility pattern from tumbling in water to straightforward moving in the ovarian fluid. Rainbow trout OF induced a trapping chemotaxis-like effect on activated male gametes and this effect depended on the properties of the activating media. The interaction of the sper-matozoa with the attracting agents was accompanied by their 'turn-and-run' behavior involving asymmetric flagellar beating and Ca2+ concentration bursts in the bent flagella segment, characteristic for the chemotactic response. Collectively, the ovarian fluid creates the optimal environment for rainbow trout spermatozoa performance, being an effective promoter of fertilization. The individual peculiarities of the egg (ovarian fluid)-sperm interaction in rainbow trout reflect the specific features of the spawning process in this species.

The Divergent Key Residues of Two Agrobacterium fabrum (tumefaciens) CheY Paralogs Play a Key Role in Distinguishing Their Functions

The chemotactic response regulator CheY, when phosphorylated by the phosphoryl group from phosphorylated CheA, can bind to the motor switch complex to control the flagellar motor rotation. Agrobacterium fabrum (previous name: Agrobacterium tumefaciens), a phytopathogen, carries two paralogous cheY genes, cheY1 and cheY2. The functional difference of two paralogous CheYs remains unclear. Three cheY-deletion mutants were constructed to test the effects of two CheYs on the chemotaxis of A. fabrum. Phenotypes of three cheY-deletion mutants show that deletion of each cheY significantly affects the chemotactic response, but cheY2-deletion possesses more prominent effects on the chemotactic migration and swimming pattern of A. fabrum than does cheY1-deletion. CheA-dependent cellular localization of two CheY paralogs and in vitro pull-down of two CheY paralogs by FliM demonstrate that the distinct roles of two CheY paralogs arise mainly from the differentiation of their binding affinities for the motor switch component FliM, agreeing with the divergence of the key residues on the motor-binding surface involved in the interaction with FliM. The single respective replacements of key residues R93 and A109 on the motor-binding surface of CheY2 by alanine (A) and valine (V), the corresponding residues of CheY1, significantly enhanced the function of CheY2 in regulating the chemotactic response of A. fabrum CheY-deficient mutant Δy to nutrient substances and host attractants. These results conclude that the divergence of the key residues in the functional subdomain is the decisive factor of functional differentiation of these two CheY homologs and protein function may be improved by the substitution of the divergent key residues in the functional domain for the corresponding residues of its paralogs. This finding will help us to better understand how paralogous proteins sub-functionalize. In addition, the acquirement of two CheY2 variants, whose chemotactic response functions are significantly improved, will be very useful for us to further explore the mechanism of CheY to bind and regulate the flagellar motor and the role of chemotaxis in the pathogenicity of A. fabrum.

Chemotaxis Toward Crude Oil by an Oil-Degrading Pseudomonas aeruginosa 6-1B Strain

The chemotactic properties of an oil-degrading Pseudomonas aeruginosa strain 6-1B, isolated from Daqing Oilfield, China, have been investigated. The strain 6-1B could grow well in crude oil with a specific rhamnolipid biosurfactant production. Furthermore, it exhibits chemotaxis toward various substrates, including glycine, glycerol, glucose, and sucrose. Compared with another oil-degrading strain, T7-2, the strain 6-1B presented a better chemotactic response towards crude oil and its vital component, n-alkenes. Based on the observed distribution of the strain 6-1B cells around the oil droplet in the chemotactic assays, the potential chemotaxis process of bacteria toward crude oil could be summarized in the following steps: searching, moving and consuming.

The role of microglia membrane potential in chemotaxis

Microglia react to danger signals by rapid and targeted extension of cellular processes towards the source of the signal. This positive chemotactic response is accompanied by a hyperpolarization of the microglia membrane. Here, we show that optogenetic depolarization of microglia has little effect on baseline motility, but significantly slows down the chemotactic response. Reducing the extracellular Ca2+ concentration mimics the effect of optogenetic depolarization. As the membrane potential sets the driving force for Ca2+ entry, hyperpolarization is an integral part of rapid stimulus-response coupling in microglia. Compared to typical excitable cells such as neurons, the sign of the activating response is inverted in microglia, leading to inhibition by depolarizing channelrhodopsins.

Chemotactic Response of Vibrio coralliilyticus to mucus from various coral species

<i>Vibrio coralliilyticus</i>, a prominent pathogenic bacteria, is known to cause tissue damage in the coral <i>Pocillopora damicornis</i> and is attracted towards the coral via chemotaxis. However, the potential of <i>V. coralliilyticus</i> to infect most of the other coral hosts via chemotaxis is unknown. The present study used capillary assays to quantify the chemotactic response of <i>V. coralliilyticus</i> to the mucus of four tank-cultivated corals, <i>Cataphyllia jardine</i>, <i>Mussidae</i> sp., <i>Nemenzophyllia turbida </i>and <i>Euphyllia ancora</i> and mucus from three wild corals, <i>Acropora</i> sp., <i>Porites</i> sp. & <i>Montipora</i> sp. The bacteria showed positive chemotactic response to each coral mucus tested, with the highest response recorded to the mucus of <i>Acropora</i> sp and the lowest response to the mucus of <i>Montipora</i> sp. A microfluidic chip was then used to assess the chemotactic preference of <i>V. coralliilyticus </i>to the mucus of the tank cultivated corals. Here too, the bacteria showed positive response with a slightly different ranking order. The strong chemotactic response of <i>V. coralliilyticus</i> towards the mucus tested could indicate a broader host range of <i>V. coralliilyticus</i> and in extension its threat to weakened coral reefs worldwide.

The Role of Microglia Membrane Potential in Chemotaxis

Microglia react to danger signals by rapid and targeted extension of cellular processes towards the source of the signal. This positive chemotactic response is accompanied by a hyperpolarization of the microglia membrane. Here we show that optogenetic depolarization of microglia has little effect on baseline motility, but significantly slows down the chemotactic response. Reducing the extracellular Ca2+ concentration mimics the effect of optogenetic depolarization. As the membrane potential sets the driving force for Ca2+ entry, hyperpolarization is an integral part of rapid stimulus-response coupling in microglia. Compared to typical excitable cells such as neurons, the sign of the activating response is inverted in microglia, leading to inhibition by depolarizing channelrhodopsins.

The role of microglia membrane potential in chemotaxis

Microglia react to danger signals by rapid and targeted extension of cellular processes towards the source of the signal. This positive chemotactic response is accompanied by a hyperpolarization of the microglia membrane. Here we show that optogenetic depolarization of microglia has little effect on baseline motility, but significantly slows down the chemotactic response. Reducing the extracellular Ca2+ concentration mimics the effect of optogenetic depolarization. As the membrane potential sets the driving force for Ca2+ entry, hyperpolarization is an integral part of rapid stimulus-response coupling in microglia. Compared to typical excitable cells such as neurons, the sign of the activating response is inverted in microglia, leading to inhibition by depolarizing channelrhodopsins.