Short term memoryCHEMOTAXIS (Random biased movement in response to a chemical)E.coli has several chemoreceptors that detect attractants and repellents. Chemoreceptors are methyl-accepting chemotaxis proteins (MCP) which specifically recognize these chemicals. MCPs are buried in the plasma membrane, facing both the cytoplasm and the periplasmic space. Besides the MCPs, several other cytoplasmic proteins (Che A, Che B,Che R, Che W, Che Y and Che Z) participate in chemotaxis.1. Attractants may either bind to MCPs directly or via another periplasmic protein.2. MCPs interact with 2 proteins, Che W and Che A on its cytoplasmic side. Che W attaches itself to MCP and helps Che A bind to it. Together, these make a complex of MCP dimer, 2 Che W monomers, and a Che A dimer. 3. Che A (a protein kinase), autophosphorylates itself in the absence of an attractant. Presence of an attractant inhibits this autophosphorylation.4. Presence of an attractant results in decreased levels of phosphorylated Che A, [Che A(P)], and in increased methylation of MCPs.5. Che R is a regulatory protein. Che R uses S-adenosyl methionine (SAM) to catalyze the methylation of MCPs. Che R, by itself, is not affected by the presence or absence of the attractant. How ever, the attractant - MCP complex is a good substrate for Che R, but NOT for phosphorylated Che B (a methlyesterase).6. Phosphorylated Che A passes on its phosphate (P) to either Che B or to Che Y.7. When Che Y is phosphorylated, [Che Y(P)], the flagella moves in the clockwise (CW)direction, resulting in tumbling. When Che Y is not phosphorylated the flagella moves in the counter clockwise (CCW) direction resulting in “RUN”.i.e. Low levels of attractant results in tumbling of the cell, where as high levels result in “RUN”.8. Irrespective of the amount of the attractant, Che Z removes phosphate (P) from phosphorylated Che Y, [ Che Y(P)], every 10 seconds. The short life time of Che Y (P) allows the bacterium to be highly responsive to changes in the concentration of the attractant.Short term memory9. The cell regulates its chemotactic response by adjusting to the level of the attractant. For this it uses Che B. Phosphorylated Che B, [Che B (P)], acts as a methylesterase and removes methyl (-CH3) group from methylated MCP.10. In the presence of an attractant, MCP is bound to the attractant and is methylated. Also, the presence of attractant results in low levels of phosphorylated Che A, [Che A (P)]. This results in both Che Y and Che B to be unphosphorylated. This is reflected by the flagella moving in the CCW direction, i.e., the cell is in the “RUN” mode.11. Increased levels of methylated MCP changes the conformation of MCP and it is againable to maintain intermediate levels of phosphorylated Che A.12. Intermediate levels of Che A (P) result in intermediate levels of Che Y(P) and Che B(P) in the cell, translating into the normal run-tumble mode for the