6 - Flagella, Motility & Taxis

Question 1

Types of prokaryote motility

Answer 1

• Flagella
• Gliding
• Twitching

Question 2

Flagella

Answer 2

Helical, thread-like appendages extending from the plasma membrane and cell wall that enable motility

Question 3

Types of flagellar arrangement

Answer 3

• Monotrichous (single, polar flagellum)
• Amphitrichous (single flagella at opposite ends of cell)
• Lophotrichous (Flagella cluster at one or both ends)
• Peritrichous (flagella spread over cell-surface)

Question 4

Three main parts of flagella

Answer 4

• Filament
• Hook
• Basal body

Question 5

Filament

Answer 5

• Hollow, rigid cylinder composed of flagellin protein molecules
• Synthesised by transfer of flagellin subunits through the hollow filament

Question 6

Flagella genes

Answer 6

• Over 50 genes control flagella synthesis and function
• Half encode structural components
• The rest control regulation of flagella assembly

Question 7

3 main flagella genes

Answer 7

fla, fli and flg

Question 8

Sheaths

Answer 8

Membraneous or lipopolysaccharide surrounding the filament

Question 9

Hook

Answer 9

Composed of protein and acts as a flexible coupling between the filament and basal body

Question 10

Basal body

Answer 10

• Attaches flagellum to the cell
• Functions as the flagellum motor
• Consists of rings attached to a central rod

Question 11

Basal body of gram negative bacteria

Answer 11

• Four rings surround central rod
• L ring (embedded in
  lipopolysaccharide)
• P ring (embedded in
  peptidoglycan)
• MS ring (embedded in
  plasma membrane)
• C ring attaches to the MS
  ring on the cytoplasmic side

Question 12

P & L ring function

Answer 12

• Act like bearings
• Gap between them and rod allows rod to rotate

Question 13

MS and C rings function

Answer 13

• Act as rotor
• Rotate within the membrane and turn rod attached to them, and the
  filament attached to the rod

Question 14

Basal body of gram positive bacteria

Answer 14

• Two rings connected to the central rod
• An inner ring connected to the plasma membrane
• An outer ring attached to peptidoglycan

Question 15

Proteins are associated with the basal body

Answer 15

• Mot A
• Mot B
• Fli Proteins

Question 16

Mot A and B proteins

Answer 16

• Provide a proton channel through the plasma membrane and rotate the flagellum
• Mot B also anchors the complex to peptidoglycan layer

Question 17

Fli proteins

Answer 17

• Important in generating flageller rotation
• Also involved in switching the direction of rotation

Question 18

Flagellar rotation

Answer 18

• Filament rotation acts like
  the propeller of a boat
• The bacterial cell rotates if the flagellum tip is artificially tethered
• Proton Motive Force drives rotation

Question 19

Proton Motive Force rotation

Answer 19

• Protons move down a charge and pH gradient
• H+ flow occurs at Mot protein channel
• Entry of a proton into the channel gives a small push
• Protons exert electrostatic forces on charges present in C and MS rings
• Rotation speed is proportional to the PMF

Question 20

How many protons required for one complete rotation

Answer 20

~1000

Question 21

Peritrichous flagella movement

Answer 21

• Counterclockwise rotation causes running
• Clockwise rotation causes tumbling (disrupts a run)
• Cells move off in new direction

Question 22

Polar flagella movement

Answer 22

• Cells move forward or backward by reversing flagellar rotation
• CCW rotation pushes, CW rotation pulls the cell

Question 23

Flagellar movement in a homogeneous environment

Answer 23

• The direction of bacterial motility is random
• Straight runs are interspersed with tumbles that result in random directional changes when running resumes

Question 24

Taxis responses in prokaryotes

Answer 24

Directed movement to attractants and from repellents

Question 25

Chemotaxis

Answer 25

- Involves biased random walk - Longer runs occur moving up a nutrient gradient towards attractant (tumbling occurs less frequently)

Question 26

Chemotaxis mechanism

Answer 26

- Prokaryotic cells sense concentration changes over time - Changes are detected through periodic (temporal) sampling of the environment - Chemoreceptors called MCPs detect attractants and repellents - Complex two component phosphorelay system regulates directional rotation of flagella

Question 27

What does MCPs stand for

Answer 27

Methyl-accepting chemotaxis proteins

Question 28

Where are MCPs found

Answer 28

In plasma membrane

Question 29

MCPs

Answer 29

- MCPs bind chemicals directly or indirectly via periplasmic binding proteins - Some MPCs can detect both attractants and repellants - Detect one or more ‘types’ of attractant (e.g. specific amino acids/sugars/light)

Question 30

Two component phosphorelay system that regulates directional rotation

Answer 30

- Involves sensor kinase CheA and response regulator CheY - If attractant levels decrease (cell moving away), less attractant bound, MCPs activate CheA to increase rate of autophosphorylation and CheA-P transfers P to CheY - Phosphorylated CheY-P diffuses to flagellar motor, causes clockwise flagella rotation (tumbling) - If attractant levels increase (cell moving towards it), converse occurs (runs) - CheY-P interacts with flagellar motor to induce CW rotation (tumbling), while CheY cannot bind to flagellar motor - Tumbling does not continue indefinitely: after a few secs the phosphoryl group is removed from CheY-P by CheZ protein, thus CCW rotation is restored

Question 31

MCP clusters

Answer 31

- MCPs form clusters associated with CheA and CheW proteins - Seen at either end of cell - MCP clusters work cooperatively to modulate CheA activity

Question 32

Adaptation

Answer 32

- Once cell has successfully responded to an attractant, it must stop responding and reset the sensory system to await further signals - Involves the methylation of MCPs

Question 33

Methylation of MCPs

Answer 33

- 1-5 methyl groups can be added to MCPs - Gradually reducing response (when fully methylated they no longer respond to attractants) - Despite the continued presence of attractant, CheA activity is eventually restored to the same value as it had in the absence of attractant - Methylation and demethylation of MCPs occurs more slowly than the phosphorylation of CheA and CheY

Question 34

In adaptation, is the cell detecting the absolute concentration of a chemical?

Answer 34

No, it is monitoring small changes in the concentrations of both attractants (and repellants) over time

Question 35

Phototaxis

Answer 35

Taxis response to light

Question 36

Aerotaxis

Answer 36

Taxis response to oxygen

Question 37

Magnetotaxis

Answer 37

Certain gram negative microaerophilic and anaerobic bacteria contain certain membrane bound magnetite or magnetosomes

Question 38

Original hypothesis for the function of magnetotaxis

Answer 38

- Magnetosomes function like compass needle, orientating bacteria along Earth's magnetic field - Magnetosomes thus enable microaerophilic and anaerobic bacteria near magnetic poles to swim down towards the sediment, to microaerophilic or anoxic conditions

Question 39

Current hypothesis for the function of magnetotaxis

Answer 39

- Magnetotaxis functions in conjunction with aerotaxis - Cells orient along magnetic field, then respond to O2 levels - Can swim in both up and down directions (CW pull or CCW push) - Reach their preferred O2 concentration (the OATZ)