Lecture 14 PMB

Question	Answer
chemotaxis	"controlling the frequency of runs and tumbles to move toward attractants or away from repellents" AND "the movement of a cell toward or away from a chemical signal, amino acids, sugars, oxygen, electron acceptors"
Peritrichous CCW rotation	multiple flagella form a bundle and work together to propel the cell forward "running"
Peritrichous CW rotation	individual filaments are pushed out of the bundle and stop working together "tumbling" reorients the cell randomly
How cells with a single flagellum change direction	either go from CCW to CW, or stop movement, reorient, and continue with same rotation cell is reoriented randomly
How to test chemotactic ability of mutants	use a soft agar and a compound that attracts bacteria + can be metabolized --> cells at center metabolize the compound and swim out in a circle up the gradient they have created
baseline behavior of cells	alternate between runs and tumbles by switching flagellar direction, usually occurs in homogenous environment
behavior of cells in presence of attractant	keep flagellum rotating in same direction, bias towards runs
Why study chemotaxis?	1) we can make and study a huge number of mutations that do not kill the cell! 2) signaling is incredibly sensitive 3) signaling is effective over a wide range of attractant concentrations --> 2 & 3 let cells "see" a better environment
How do cells sense changes in levels of attractants/repellents and transmit info to flagellum?	have MCP sensor proteins at the poles external domain binds attractant in periplasm, HAMP domain transmits info of ligand binding, methylation region functions in adaptation, signaling region binds CheW and CheA --> relay info to downstream proteins
CheA	Histidine Kinase that binds to signaling region of MCP if attractants bind, it loses its phosphate group will form trimers with CheW
CheY	downstream protein of MCP pathway that acts as a response regulator
CheZ	dephosphorylates CheY-P (thus deactivating it)
How do we get more runs than tumbles when more of an attractant binds?	1) less CheA activity --> less CheA-P 2) Less of CheY-P 3) Less of CheY-P binding to FliM on flagellum 4) CheZ deactivates CheY --> cause initial flagellar response to stimuli --> less CW rotation, less tumbles VICE VERSA FOR DECREASE IN ATTRACTANT
How does a bacteria know if it is swimming up a gradient of an attractant?	not by spatial sensing but instead by TEMPORAL sensing --> to do this they must reset signal system rapidly so that they can see if attractant concentration increases or decreases with respect to current level
CheA as a function of the amount of attractant outside the cell	NOT a linear function because this is not a response to the absolute level, but instead the CHANGE in levels of the attractant --> requires adaptation to the ambient level for every change in environment
Additional steps of adaptation if INCREASE in attractant levels	1) CheR (always active) methylates MCP 2) CheA phosphorylates CheB LESS 3) CheB (response regulator) demethylates MCP LESS --> cause buildup of methyl groups on MCP --> INCREASE ability to stimulate CheA --> return to pre-stimulus levels
Additional steps of adaptation if DECREASE in attractant levels	1) CheR (always active) methylates MCP 2) CheA phosphorylates CheB MORE 3) CheB (response regulator) demethylates MCP MORE --> cause depletion of methyl groups on MCP --> DECREASE ability to stimulate CheA --> return to pre-stimulus levels
summary of chemotaxis	more attractant bind --> less CheA-P and less CheY-P more CheY-P binding to FliM --> more tumbling more methyl groups on MCP --> more ability to stimulate CheA phosphorylation less methyl groups on MCP --> less ability to stimulate CheA phosphorylation
CheB	methylesterase that REMOVES methyl groups from MCPs and reduces their ability to activate CheA
CheR	methyltransferase that constantly adds methyl groups to specific sites on MCPs, increasing their ability to activate CheA

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	Created by Candice Young over 6 years ago