Bacterial chemotaxis is the movement of bacteria towards or away from chemical stimuli in the surrounding media. Bacteria respond to chemotactic signals through chemoreceptors which bind specific ligands and transduce signals through a modified two-component system. Typical chemoreceptors bind a ligand in the periplasm and signal across the inner membrane to the cytoplasmic chemosensory array through the inner membrane. Bacterial chemoreceptors must integrate multiple signals within an array of different receptor homologues to a single output. Chemoreceptors act cooperatively to allow a rapid signal spread across the array and large signal gain. Chemoreceptors adapt to a signal by chemical modification of their cytoplasmic domains in order respond across a wide range of effector concentrations. How bacterial chemoreceptors transduce signals through the inner membrane, integrate multiple effector responses, signal cooperatively and adapt to result in a single output signal is not currently fully known. In Rhodobacter sphaeroides, additional complexity arises from the presence of multiple homologues of various chemotactic components, notably the array scaffold protein CheW. Decoding this signalling mechanism and heterogeneity involved in this system is important in decoding the action of a biological system, with implications for biotechnology and synthetic biology. This study used the two model systems Escherichia coli and R. sphaeroides to analyse the mechanism of signalling through bacterial chemoreceptors. Rational design of activity-shifting chemoreceptor mutations was undertaken and these variants were analysed in phenotypic and fluorescence localisation studies. Molecular-dynamics simulations showed an increase in flexibility of chemoreceptors corresponds to a decrease in kinase output activity, which was determined by the computational tracking of bacteria free-swimming in media. Fluorescence recovery after photobleaching was used to show that this increase in flexibility results in a decrease in binding of receptors to their array scaffold proteins. A two-hybrid screen also suggested that inter-receptor affinity is also likely to decrease. These results show that signalling through chemoreceptors is likely through a mechanism involving the selective flexibility of chemoreceptor cytoplasmic domains. Analysis of R. sphaeroides chemoreceptors and CheW scaffold proteins in E. coli showed that it should be possible to design, from the bottom-up, a functional bacterial chemotaxis system in order to analyse individual protein specificity. Expression of R. sphaeroides MCPs in this E. coli system show the reconstitution of a chemotactic array, but not one capable of signalling specifically to proposed attractants. Results gained from this system suggest the R. sphaeroides CheW proteins are not homologous and their differential binding affinities may allow array activity 'fine-tuning'.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:658484 |
| Date | January 2014 |
| Creators | Allen, James Robert |
| Contributors | Armitage, Judith |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:ce7de07a-dee6-471b-9f70-22714e617693 |
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