Editing Chemotaxis (section)

===Signal transduction===
[[Image:ChtxAspRec.png|right|350 px|<div style="text-align: center;border:none">Domain structure of chemotaxis receptor for Asp</div>]]
Chemical gradients are sensed through multiple [[transmembrane receptor]]s, called methyl-accepting chemotaxis proteins (MCPs), which vary in the molecules that they detect.<ref name=":1"/> Thousands of MCP receptors are known to be encoded across the bacterial kingdom.<ref>{{cite journal | vauthors = Galperin MY | title = A census of membrane-bound and intracellular signal transduction proteins in bacteria: bacterial IQ, extroverts and introverts | journal = BMC Microbiology | volume = 5 | pages = 35 | date = June 2005 | pmid = 15955239 | pmc = 1183210 | doi = 10.1186/1471-2180-5-35 | doi-access = free }}</ref> These receptors may bind attractants or repellents directly or indirectly through interaction with proteins of [[periplasmatic space]].<ref>{{Cite book| vauthors = Auletta G |title=Cognitive Biology: Dealing with Information from Bacteria to Minds|publisher=Oxford University Press|year=2011|isbn=978-0-19-960848-5|location=United States|pages=266}}</ref> The signals from these receptors are transmitted across the [[plasma membrane]] into the [[cytosol]], where ''[[Che proteins]]'' are activated.<ref name="ReferenceA">{{cite journal | vauthors = Falke JJ, Bass RB, Butler SL, Chervitz SA, Danielson MA | title = The two-component signaling pathway of bacterial chemotaxis: a molecular view of signal transduction by receptors, kinases, and adaptation enzymes | journal = Annual Review of Cell and Developmental Biology | volume = 13 | pages = 457–512 | date = 1997 | pmid = 9442881 | pmc = 2899694 | doi = 10.1146/annurev.cellbio.13.1.457 }}</ref> The Che proteins alter the tumbling frequency, and alter the receptors.<ref name="ReferenceA"/>

====Flagellum regulation====
The proteins CheW and CheA bind to the receptor. The absence of receptor activation results in [[phosphorylation|autophosphorylation]] in the [[histidine kinase]], CheA, at a single highly conserved histidine residue.<ref>{{cite web|url=https://www.youtube.com/watch?v=h4lv7cBYVug| archive-url=https://web.archive.org/web/20150711181802/https://www.youtube.com/watch?v=h4lv7cBYVug| archive-date=2015-07-11 | url-status=dead|title=Chemotaxis|last=ToxCafe|date=2 June 2011|access-date=March 23, 2017|via=YouTube}}</ref>{{better source needed|date=March 2017}} CheA, in turn, transfers phosphoryl groups to conserved aspartate residues in the response regulators CheB and CheY; CheA is a histidine kinase and it does not actively transfer the phosphoryl group, rather, the response regulator [[Protein-glutamate methylesterase|CheB]] takes the phosphoryl group from CheA.{{citation needed|date=March 2017}} This mechanism of signal transduction is called a [[two-component regulatory system|two-component system]], and it is a common form of signal transduction in bacteria.{{citation needed|date=March 2017}} CheY induces tumbling by interacting with the flagellar switch protein FliM, inducing a change from counter-clockwise to clockwise rotation of the flagellum. Change in the rotation state of a single flagellum can disrupt the entire flagella bundle and cause a tumble.{{citation needed|date=March 2017}}

====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>