Bacterial Chemotaxis

Hello again! Today I'm going to tackle a broad, essay type question that pertains to bacterial movement: Chemotaxis.

Chemotaxis is the ability of a bacterium to move along a concentration gradient, either towards an attractant or away from a repellent. The attractant or repellent is termed a chemoeffector, and is monitored by a system of transmembrane sensor proteins, called methyl-accepting chemotaxis proteins (MCP), or receptor-transducer proteins. These proteins affect a two component system: CheA, a cytoplasmic histidine kinase, and CheY, a response regulator. Action upon this system affects the flagellar motor.

Bacteria swim by rotating flagella. Counter-clockwise rotation align the flagella in a single bundle, causing the bacterium to swim in a straight line (termed a "run"). Clockwise rotation causes this flagellar bundle to break apart, and results in random tumbling in place (termed a "tumble"). As few as 25% of the flagella need to rotate clockwise to cause random tumbling, but the more flagella rotating in this manner, the greater the change of direction.

Bacteria are unable to choose the direction in which they swim, and are unable to swim in a straight line (a run) for very long due to rotational diffusion; they "forget" which direction they were going. This results in random run and tumble movement across space. Chemoeffectors influence this random movement. When a bacterium senses it is going towards an attractant or away from a repellent (the "correct" direction from the bacterium's point of view), it will swim in a straight line for longer; this results in a longer run vs tumble phase. The presence of an attractant decreases the probability of clockwise rotation of flagella, keeping the bacterium from tumbling. The presence of a repellent increases the probability of clockwise flagellar rotation, resulting in a shorter run, and more change of direction. Therefore, attractants see longer, more frequent runs mixed with shorter, less frequent tumbles, resulting in an overall movement towards the attractant (or, conversely, away from the repellent).

Bacteria sense chemoeffectors on a temporal gradient: they are able to remember past concentrations long enough to compare them to present concentrations, and then use this information to make a decision. This memory is long enough for the bacteria to compare two points more distal than its body length, yet short enough to signal the bacteria before it tumbles randomly.

Six genes are required for chemotaxis: CheA, CheB, CheR, CheW, CheY, and CheZ. In mutants that have any of of those genes knocked out, chemotaxis is impossible. As mentioned above, chemotaxis is controlled by a two-component system, which is alerted by methyl-accepting chemotaxis proteins that span the membrane and monitor chemoeffectors in the periplasmic space. CheY is ultimately responsible for the way in which a flagellar motor turns. If it attaches to proteins in the flagellar motor (FliM), then the motor will turn clockwise. If it doesn't, the motor turns counterclockwise. Therefore, CheY must attach to the flagellar motor to cause tumbling. CheY is activated by accepting a phosphate group from CheA. CheA is signaled by transmembrane proteins, of which there are 5: Tar (taxis to aspartate and away from repellents), Trg (taxis to ribose, glucose and galactose), Tap (taxis to dipeptides), Tsr (taxis to serine and away from repellents) and Aer (taxis to oxygen as it oxidizes FADH to FAD). The presence of these substances in the extra cellular space causes a conformational change in the transmembrane protein. This initiates a CheW mediated response in CheA phosphorilation:

CheA + ATP=CheA-P + ADP + Pi

CheA then phosphorilates CheY: CheA-P + CheY=CheA + CheY-P

The binding of CheY-P to the flagellar motor causes clockwise rotation. If CheY-P is dephosphorilated, then it will not bind and the flagellar motor will turn counter-clockwise. CheZ is responsible for the dephosphorilation of CheY-P in the cytoplasm. Under normal circumstance, CheY is phosphorilated and dephosphorilated at a constant rate, allowing for the random run/tumble action observed in bacteria not experiencing chemotaxis. When an attractant is sensed, autophosphorilation of CheA is decreased, which decreases the phosphorilation of CheY, and therefore the probability of flagella turning clockwise. When a repellant is sensed, the exact opposite happens.

CheA also phosphorilated CheB, which is a regulator that governs adaptation to a particular concentration of attractant. CheB-P removes methyl groups from glutamate residues in the receptor-transducer proteins. CheR adds methyl groups to the glutamate residues. When an attractant is present, CheA does not become phosphorilated, and therefore CheB does not phosphorilate either. CheB cannot remove methyl groups from the glutamate residues. Higher methylation of glutamate residues stimulate tumbling, and therefore the chemotaxis stops for this concentration of attractant. When the concentration changes again, chemotaxis returns as normal.