13. Bacterial Chemotaxis

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cornpops
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107653
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13. Bacterial Chemotaxis
Updated:
2011-11-08 01:57:13
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PMB 112 Midterm2
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general microbiology midterm 2
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  1. chemotaxis
    • the movement of a cell toward or away from a chemical signal
    • - amino acids, sugars, oxygen, electron acceptors
  2. swarming assay
    • soft agar + compound that attracts the bacteria and can be metabolized
    • cells inoculated at center metabolize the compound and swim out in circle up the gradient they have created
    • assay only works if bacteria can metabolize the attractant
    • takes hours, includes growth

    • agarose plug containing attractant is placed in center of liquid suspension of bacteria
    • cells accumulate in bright ring surrounding the plug
    • does not require growth, response in minutes
  3. 3-D random walk of chemotaxis
    • CCW- run
    • CW- tumble

    swimming toward greater concentration of attractant - biased towards CCW for longer times
  4. Why study chemotaxis?
    • mutants have clearly observable phenotypes, but genes are not usually essential
    • chemotaxis signaling is incredibly sensitive-cells can respond effectively to very small changes (<1%) in attractant concentration
    • chemotaxis signaling is effective over a wide range of attractant concentrations
    • - enable bacteria to sense slightly better environment over a wide range of backgrounds
  5. relationship between attractant binding and CheA~P
    ↑ attractant binding → ↓CheA~P
  6. basal level of attractant
    • basal level of attractant binding to MCP
    • basal CheA activity level, making CheA~P
    • basal level of CheY~P
    • basal level of CheY~P binding to FliM on flagellum

    create a baseline level of CheY~P binding to FliM, with flagellar reversal occurring every 2-4 seconds
  7. attraction concentration increase
    • increased attractant binding to MCP
    • decreased CheA activity, making less CheA~P
    • less phosphotransfer to CheY, making less CheY~P
    • less CheY~P binding to FliM on flagellum
    • CheZ deactivates CheY

    effect is less CW rotation, fewer tumbles, longer runs
  8. attraction concentration decrease
    • decreased attractant binding to MCP
    • increased CheA activity, making more CheA~P
    • more phosphotransfer to CheY, making more CheY~P
    • more CheY~P binding to FliM on flagellum
    • CheZ deactivates CheY

    effect is more CW rotation, more tumbles, shorter runs
  9. How does a bacterium know if it is swimming up a gradient of attractant?
    • not by spatial sensing
    • use a temporal sensing mechanism (more signal now than there was a few seconds ago?)
    • to compare now to a few seconds ago, bacteria reset their signaling system rapidly - current attractant concentration they currently see becomes the new norm
  10. adaptation
    • resetting the signalling baseline to reflect current conditions
    • return of CheA activity to pre-stimulus level
  11. basal level of attractant adaptation
    • basal level of attractant binding to MCP
    • basal CheA activity level, making CheA~P
    • basal level of CheY~P
    • basal level of CheY~P binding to FliM
    • CheZ deactivates CheY
    • CheR methylates MCP
    • CheA phosphorylates CheB
    • CheB demethylates MCP
  12. attraction concentration increase adaptation
    • increased attractant binding to MCP
    • decreased CheA activity level, making less CheA~P
    • less CheY~P
    • less CheY~P binding to FliM
    • CheZ deactivates CheY
    • CheR methylates MCP
    • CheA phophorylates CheB less
    • CheB demethylates MCP less

    • 1-5 create the initial response
    • 6-8 cause buildup of methyl groups on MCP -> increases its ability to stimulate CheA
    • causes adaptation
  13. attraction concentration decrease adaptation
    • decreased attractant binding to MCP
    • increased CheA activity level, making more CheA~P
    • more CheY~P
    • more CheY~P binding to FliM
    • CheZ deactivates CheY
    • CheR methylates MCP
    • CheA phosphorylates CheB more
    • CheB demethylates MCP more

    • 1-5 create the initial response to stimulus
    • 6-8 cause depletion of methyl groups on MCP -> decreases the ability to stimulate CheA
    • causes adaption
  14. more attractant binding
    less CheA~P and less CheY~P
  15. more CheY~P binding to FliM
    more tumbling
  16. more methyl groups on MCP
    more ability to stimulate CheA phosphorylation
  17. fewer methyl groups on MCP
    less ability to stimulate CheA autophosphorylation
  18. CheA activity in response to attractant concentration
    • the cell shouldn't respond to the absolute level of attractant
    • responds to changes in the amount of attractant
    • requires adaption to the ambient level

    • when attractant level is increased (stimulus)
    • rapid decrease in CheA activity (response)
    • if cells held at increased concentration, CheA activity returns to pre-stimulus level (adaptation)

    reverse is true for rapid decrease in amount of attractant
  19. How does the chemotaxis signaling pathway adapt to the current attractant concentration?
    CheR constantly adds methyl groups to specific sites on the MCPs -> increases the ability of the MCP to activate CheA at any [attractant]

    CheB removes methyl groups from the MCPs -> reducing their ability to activate CheA at any [attractant]. CheB is a response regulator whose receiver domain is phosphorylated by CheA - increases the activity of CheB. CheB is activated at the same time as they CheY pathway
  20. evidence that receptors cooperate with each other, rather than working in isolation
    the presence of one amino acid can increase the cell's response to a different amino acid
  21. trimers of MCPS found in polar caps
    • CheW and CheA link the trimers into tight clusters that can influence each other's signalling
    • attractant binding by one trimer can sensitize other trimers

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