Editing Chemotaxis (section)

====Receptor regulation====
[[Image:Chtxbactsign1.png|right|450 px|<div style="text-align: center;border:none">Signalling pathways of E.coli</div>]]
CheB, when activated by CheA, acts as a [[demethylase|methylesterase]], removing methyl groups from [[glutamate]] residues on the [[cytosol]]ic side of the receptor; it works antagonistically with CheR, a methyl[[transferase]], which adds methyl residues to the same glutamate residues.<ref name=":1" /> If the level of an attractant remains high, the level of phosphorylation of CheA (and, therefore, CheY and CheB) will remain low, the cell will swim smoothly, and the level of methylation of the MCPs will increase (because CheB-P is not present to demethylate).<ref name=":1">{{cite journal | vauthors = Wadhams GH, Armitage JP | title = Making sense of it all: bacterial chemotaxis | journal = Nature Reviews. Molecular Cell Biology | volume = 5 | issue = 12 | pages = 1024–1037 | date = December 2004 | pmid = 15573139 | doi = 10.1038/nrm1524 | s2cid = 205493118 }}</ref> The MCPs no longer respond to the attractant when they are fully methylated; therefore, even though the level of attractant might remain high, the level of CheA-P (and CheB-P) increases and the cell begins to tumble.<ref name=":1"/> The MCPs can be demethylated by CheB-P, and, when this happens, the receptors can once again respond to attractants.<ref name=":1" /> The situation is the opposite with regard to repellents: fully methylated MCPs respond best to repellents, while least-methylated MCPs respond worst to repellents.{{citation needed|date=March 2017}} This regulation allows the bacterium to 'remember' chemical concentrations from the recent past, a few seconds, and compare them to those it is currently experiencing, thus 'know' whether it is traveling up or down a gradient.
<ref>{{Cite book| vauthors = Shu C, Chen PC, Fung YC |title=An Introductory Text to Bioengineering (Advanced Series in Biomechanics - Vol. 4)|publisher=World Scientific Publishing Co. Pte. Ltd.|year=2008|isbn=9789812707932|location=Singapore|pages=418}}</ref> that bacteria have to chemical gradients, other mechanisms are involved in increasing the absolute value of the sensitivity on a given background. Well-established examples are the ultra-sensitive response of the motor to the CheY-P signal, and the clustering of chemoreceptors.<ref>{{cite journal | vauthors = Cluzel P, Surette M, Leibler S | s2cid = 5334523 | title = An ultrasensitive bacterial motor revealed by monitoring signaling proteins in single cells | journal = Science | volume = 287 | issue = 5458 | pages = 1652–5 | date = March 2000 | pmid = 10698740 | doi = 10.1126/science.287.5458.1652 | bibcode = 2000Sci...287.1652C }}</ref><ref>{{cite journal | vauthors = Sourjik V | title = Receptor clustering and signal processing in E. coli chemotaxis | journal = Trends in Microbiology | volume = 12 | issue = 12 | pages = 569–76 | date = December 2004 | pmid = 15539117 | doi = 10.1016/j.tim.2004.10.003 | citeseerx = 10.1.1.318.4824 }}</ref>