10.5 - Chemotaxis

Question 1

What happens in the running stage?

Answer 1

All the flagella bundle together (if it has several flagella) and rotate and the bacteria will swim in straight lines.
o Flagella counter-clockwise

Question 2

What happens in the tumbling stage?

Answer 2

The flagella will de-bundle and occupy a large space.

o Flagella clockwise

Question 3

What happens when bacteria run towards an attractant or away from a repellent?

Answer 3

They will suppress the tumbling stage so that they can run further (long run). This is what is referred to as random biased walk.

Question 4

What is CheY-P also called?

Answer 4

It is phosphorylated CheY also known as tumble factor

Question 5

What does CheY-P do?

Answer 5

Interacts with proteins in the basal body of the flagella and will promote the clockwise rotation (tumbling phase). CheY is made by default and acts as a RR protein (Response regulator).

Question 6

What is Methyl Accepting Chemotaxis Protein?

Answer 6

MCP is a transmembrane protein in the cytoplasmic membrane.These chemoreceptors cluster at one pole of E. coli, forming a “nose”

Question 7

What does CheW do?

Answer 7

Connects CheA to MCP

Question 8

What is CheA?

Answer 8

A kinase (a Histidine Protein Kinase), which is active by default

Question 9

Where are CheW and CheA found?

Answer 9

On the cytoplasmic side of the inner mb and aggregate in a tight array

Question 10

What happens when no ligand is bound to MCP?

Answer 10

Both CheA subunits are in very close proximity to each other, CheA is thus active and will therefore be able to phosphorylate CheY.

Question 11

What does phosphorylates CheY do?

Answer 11

The phosphorylated CheY will interact with the flagellar basal body and promote the clockwise rotation of the flagella (tumbling).

Question 12

What happens when ligands bind to MCP?

Answer 12

The signal is transmitted to CheW (adaptor protein) and then to the 2 CheA domains are separated (deactivated) and will not be able to phosphorylate CheY. So tumbling factor synthesis is suppressed by ligand binding

Question 13

What does CheZ do?

Answer 13

CheY that were already phosphorylated will get quickly dephosphorylated by a phosphatase called CheZ.

Question 14

When will the flagella rotate counter clockwise (running stage)?

Answer 14

Since there is no more CheY-P, the flagella will rotate counter-clockwise (running stage).

Question 15

When is the tumbling stage repressed and the running phase favored?

Answer 15

E. coli is randomly tumbling in the media, and then, by chance, it gets oriented in the right direction (towards a nutrient source for example). E. coli knowns when it is oriented in the right direction because that is when the ligand from the extracellular space binds to MCP.

Question 16

What allows bacteria to bias the random walk?

Answer 16

The bacterium makes a temporal comparison between 2 time points, T1 and T2, which allows to bias the random walk.

Question 17

What does CheR do?

Answer 17

CheR will methylate MCP. CheR adds –CH3 by default (covalent modification), making MCP less sensitive to ligands so higher ligand concentrations are needed to suppress tumbling. This methylation will desensitize MCP to the ligand (i.e. the attractant). By making the receptors less sensitive to the ligand, only a higher concentration of ligand will be able to bind to the receptors and only then will tumbling factor synthesis be inhibited, which would then enable the bacterium to run. The receptors (MCP) have been “adapted” by CheR

Question 18

When will MCP demthylate CheA?

Answer 18

Once methylated, MCP will only deactivate CheA when higher concentration of ligand is bound to it.

Question 19

What happens when there is no more attractant?

Answer 19

Proteins called CheB-P will counteract the methylation (by default) by removing the methyl group from MCP and therefore re-sensitizes it to low ligand concentrations (resets sensitive MCP). CheB is also a RR protein (response regulator). Just like CheY, CheB is also phosphorylated by CheA.

Question 20

What does demethylation of MCP do?

Answer 20

Ensures that MCP is very sensitive when there is no attractant attached to it. This ensures that E. coli can start running towards an attractant when it senses low concentrations of that attractant.

Question 21

How long is CheY-P phosphporylated for?

Answer 21

CheY-P is a short-lived protein and it is only phosphorylated for about 30 sec at a time.

Question 22

How do you get a CheY gradient?

Answer 22

It is phosphorylated at the nose and diffuses towards the basal flagellar motor, resulting in a CheY gradient in which the protein becomes inactive (bc of dephosphorylation by CheZ w/in 30 sec) and is degraded (bc it is short-lived). This is a way that enables the cell to distinguish its front from its back.