17 BIO 326R Chemotaxis March 4

17 BIO 326R Chemotaxis March 4 - Bio 326R Chemotaxis...

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Bio 326R Chemotaxis
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Flagella Monotrichous (one flagella) or peritrichous, atrichous, etc Can rotate clockwise or counterclockwise E. coli is multitrichous , but the flagella are all located at one end of the bacterium. The direction of the spin of the flagella controls movement CCW: forward CW: tumble
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Chemotaxis How does E. coli know where to swim?
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Chemotaxis In a liquid medium, E. coli shows long straight “runs” followed by short tumbles, followed by another run in a different, random direction. This is called a “random walk .” E. coli runs about 95% of the time, tumbles 5% This applies to any media – whether high or low in attractants or repellants, the bacterium will maintain the 95:5 ratio of running to tumbling How to get from A to B with only random changes in direction? If the concentration of an attractant is increased, the tumbling frequency is decreased (or the runs are longer). Decreasing concentration of attractants causes tumbling frequency to increase (i.e. the runs are shorter) Repellants work the same way but in reverse After a short time at the new concentration of attractant, the tumbling frequency re-sets to the baseline level (95:5) This is called a biased random walk . Experimentally, this kind of behavior can be seen in semi-solid media. In liquid media there is no consistent gradient. In solid media, there is no motility.
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Chemotaxis
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Methyl Accepting Chemotaxis Proteins  (MCPs) MCPs are membrane spanning proteins with a binding domain on the periplasmic side and a cytoplasmic domain. The cytoplasmic domain interacts with CheW and CheA The extracellular/periplasmic domain binds specific molecules, we’ll be talking about MCPs that bind
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