13. Bacterial Chemotaxis

• the movement of a cell toward or away from a chemical signal
• - amino acids, sugars, oxygen, electron acceptors

• 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

• CCW- run
• CW- tumble

swimming toward greater concentration of attractant - biased towards CCW for longer times

• 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

↑ attractant binding → ↓CheA~P

• 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

• 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

• 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

• 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

• resetting the signalling baseline to reflect current conditions
• return of CheA activity to pre-stimulus level

• 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

• 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

• 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

less CheA~P and less CheY~P

more tumbling

more ability to stimulate CheA phosphorylation

less ability to stimulate CheA autophosphorylation

• 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

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

the presence of one amino acid can increase the cell's response to a different amino acid

• 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