Question

How would the presence of a repellent molecule influence the direction that a bacterium would move? Explain using the various elements of the MCP-Che pathways.

Answer 1

The locomotion of a bacterium in response to a chemical compound (attractant or repellent) is known as chemotaxis.

The bacterium moves toward or in the direction of a attractant compounds like sugars, such as maltose, ribose, galactose, and amino acids such as L-aspartate and L-serine the bacterium tends to keep running in the direction of the attractant this locomotion is known as counter clockwise motion. While in th presence of a repellent compound such as toxic metals cobalt,nickel(lethal) or indole (membrane disruptive compounds) and weak acids(damage the cell wall integrity) the bacteria tends to move away and tumble this type of mobility is called as clockwise flagellar motion.

MCP-Che pathway for chemotaxis:

The MCP (methyl accepting chemotaxis proteins) function as transmembrane receptors which sense the chemical gradients. The attractant or repellents bind to the extracellular domain of the receptor and the intracellular domain starts the signal transduction across the plasma membran into the cytoplasm where they activate the Che protein. These Che proteins phosphorylate the CheY protein. The phosphorylated CheY protein controls the flagellar rotation and influence the direction of mobility. The methylation of the MCP receptor is maintained for continuous locomotion of the bacterium. The binding of the attractant causes an increase in the methylation level from the normal methylated receptor while the repellant causes demethylation of the MCP receptor decreasing the level of methylated receptors.

The MCP signalling domain are methylated by S-adenosylmethionine dependent methyltransferase(CheR) and demethylated by a conserved methyl esterase(CheB). This methylation and demethylation provide mechanism for sensory adaptation of the bacterium. The CheA kinase which phosphorylates the CheY activity is dependent on this methylation state of MCP. The presence of repellant which causes decreased levels of MCP methylation results in high CheA kinase activity while elevated methylation levels caused by the presence of attractan have the opposite effect.