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MB 414 > Bacterial Differentiation > Flashcards

Bacterial Differentiation Flashcards

1
Q

Types of differentiation

A
  1. Caulobacter
  2. Cyanobacteria (grow in chains)
  3. Myxococcus
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2
Q

Caulobacter crescentus

A
  • every cell division it undergoes is asymmetric
  • each division results in two types of cells, a motile swarmer cell and a sessile stalked cell.
  • mother cell is always the stalked form of the bacterium.
  • stalk keeps cell docked to the strata of its environment
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3
Q

swarmer cells

A
  • the daughter cell
  • will have a flagellum and is motile
  • will swim around by chemotaxing toward nutrients.
  • when it finds a nutrient rich environment, it will lost its flagella and form a holdfast (attachment organ)
  • at the same time as replacing flagellum, the cell will initiate DNA replication
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4
Q

cell division in Caulobacter

A
  • only two chromosomes made
  • after the chromosomes have been separated, a flagellum will form on the opposite pole from the holdfast, and cell division can proceed
  • only sessile cells divide.
  • cell division is asymmetric - swarmer cell is smaller than stalk cell in addition to pole structures (holdfast vs. flagella)
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5
Q

Regulation in Caulobacter

A
  • differential expression during caulobacter life cycle
    • flagella are only expressed in swarmer cells
    • chemotaxis proteins only expressed in stalked cells
    • hold-fast only expressed in stalked cells
    • DNA replication only occurs in stalked cells
    • Cell division only occurs in stalked cells.
  • all coordinated by master regulated, CtrA
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6
Q

CtrA

A
  • regulation of CtrA on 3 levels:
    • transcription
    • phosphorylation
    • proteolysis
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7
Q

Transcriptional regulation of CtrA

A
  • CtrA expressed from two promotors, P1 and P2
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8
Q

P1

A
  • only transcribed immediately after chromosome division when the DAN is hemimethylated (by ccrM methylase)
  • after DNA become fully methylated, transcription from P1 stops
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9
Q

P2

A
  • transcribes CtrA after P1

- controlled by CtrA-P (positive auto regulation)

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10
Q

ccrM methylase

A
  • recognizes GANTC

- because DNA replication is semi-conservative, new strands ill be hemi-methylated.

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11
Q

phosphorylation regulation of CtrA

A
  • CtrA must be phosphorylated in order to be active
  • there are multiple kinases that transfer a phosphate to CtrA
  • CckA (Cell Cycle Kinase) is the most important.
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12
Q

Proteolysis regulation of CtrA

A
  • two proteases ClpP and ClpX, which degrade CtrA. Turned on by CtrA-P
  • work in a complex called ClpXP
  • recognize amino acid sequence at C-terminal end of protein called receiver domain (RD)
  • work in conjunction with DivK
  • RD tagged proteins are targets for proteolysis
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13
Q

Targets of CtrA regulation

A
  • regulates generous involved in:
    • cell cycle regulation
    • flagellum biosynthesis
    • DNA methylation
    • chemotaxis
    • holdfast biosynthesis
    • ClpXP proteases
  • represses chromosome replication
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14
Q

Differential expression in Stalked vs swarmer cells

A
  • due to polar localization of the kinases CckA and DivJ, and well as phosphatase PleC.
  • CckA and PleC localize to the swarmer pole
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15
Q

CckA

A

serves to phosphorylate CtrA

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16
Q

PleC

A

serves to desphophorylate DivD and DivK

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17
Q

DivJ

A

localizes to the swarmer pole and is the kinase for DivK and PleD

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18
Q

DivK-P

A
  • controls CtrA proteolysis
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19
Q

Differential expression of flagella

A

flagella controlled by phosphorylation status of DivK and PleD

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20
Q

PleD-P

A

flagella release

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21
Q

When cell division occurs

A
  • DivK and PleD will be active in stalked cell, so CtrA will be degraded by ClpXP (dependent on DivK) and flagella will be repressed
  • Swarmer cells will contain will contain high levels of CtrA, which will continue to activate CtrA dependent genes and repress DNA replication.
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22
Q

CtrA regulation in swarmer cell. Genes on

A
  • Flagellum
  • CtrA
  • Chemotaxis
  • NO Division
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23
Q

CtrA regulation in stalked cell. Genes on

A
  • chromosome rep
  • Clp protease
  • Hold fast
  • Division proteins
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24
Q

Myxococcus xanthans

A
  • predatory bacteria
  • they eat only protein
  • hunt in packs because when they find prey, they secrete proteases that will degrade the prey and release amino acids.
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25
Q

gliding motility

A
  • do not swim, they move along the surface by gliding

- social motility and adventurous motility

26
Q

social motility

A
  • the pack will stay together
27
Q

adventurous motility

A
  • sometimes a single bacteria will break from the pack looking for food but will always return to the pack quickly.
28
Q

Starvation conditions

A
  • when they become starved, they will differentiate into a fruiting body and then sporulate
  • the fruiting body contains about 100,000 cells.
  • fruiting body makes sure all cells with sporulate together, so that when conditions improve, all the cells can germinate coordinately, since bacteria hunt in a pack.
29
Q

signaling fruiting body formation

A
  • coordinated through 6 signal pathway but most important are A signals and C signals
30
Q

A-signal

A
  • induces aggregation of bacteria
31
Q

C-signal

A
  • induces fruiting body formation
32
Q

Response to starvation

A
  • sense nutrient limitation by using the ribosome to assess the pool of loaded tRNAs
  • if a ribosome encounters a codon and cognate acyl-aa-tRNA is missing, it will transfer an orthophosphate to the 3’ end of GTP to form guanosine tetra-phosphate (ppGpp)
33
Q

ppGpp

A
  • common starvation signal in many bacteria

- required for the A-sinal in M. xanthus

34
Q

A-signal components

A
  • A Signaling Genes (ASG)
  • AsgA
  • AsgB
  • AsgC
  • AsgD
  • AsgE
35
Q

AsgA

A
  • membrane kinase
36
Q

AsgB

A
  • DNA bindig protein
37
Q

AsgC

A
  • Sigma factor
38
Q

AsgD

A
  • sensor kinase

- senses extracellular amino acid levels

39
Q

AsgE

A
  • Protease
40
Q

The A-signal itself

A
  • combination of 6 amino acids
  • Trp
  • Pro
  • Phe
  • Tyr
  • Leu
  • Ile
  • and peptides containing these amino acids.
41
Q

Sensing the A signal

A
  • sensed by two a two component regulatory circuit encoded by sasS and sasR
42
Q

sasS

A
  • senses A-signal

- membrane-bound histidine kinase, which passes the signal to sasS

43
Q

sasR

A
  • response regulator
  • will activate many genes that use an alternative sigma factor (sigma54).
  • these genes will start aggregation phase of fruiting body formation, and cells will come together into a large, flat mass.
  • formation of large 3-D fruiting body requires C-signal
44
Q

C-signal

A
  • once cells have aggregated due to A-signal, the C-signal is turned on
  • C-signal is the membrane protein CsgA
45
Q

CsgA

A
  • increases response to itself and to cell density (like quorum sensing)
  • CsgA can never stimulate a receptor on the same cell it is displayed on, and stimulation of a different cell requires close cell contact.
46
Q

C-signal propagation and A-signal crosstalk

A
  • as cell detects more CsgA from other cells, it will produce more of its own CsgA
  • done through sensing system consisting of actABCDEF
  • activates master regulator of fruiting body formation, FruA
47
Q

FruA

A
  • master regulator of fruiting body formation
  • response regulator
  • it’s transcription is regulated by the A-signal and phosphorylation regulated by the C-signal.
  • controls two pathways, Frz and Dev
48
Q

Frz

A

involved in chemotaxis and motility

49
Q

Dev

A

involved in fruiting body development and sporulation

50
Q

Once mature fruiting body is formed

A
  • some cells inside fruiting body will form spores, which will wait for external conditions to improve
  • germination of spores is done coordinately
  • internal structure of fruiting body contains spores (coccid cells)
51
Q

Why oxygenic photosynthesis and nitrogen fixation and incompatible?

A
  • The O2 produced by the water splitting enzyme of photosystem II will bind to and irreversibly inactivate the nitrogenase active site.
52
Q

How cyanobacteria get past this

A
  • grow in chains, and one in ten will differentiate into a heterocyst
53
Q

heterocyst

A
  • can fix nitrogen an anaerobic environment, and the fixed nitrogen (in form of amino acids) is shared with neighboring cells through channels between the cells.
  • the heterocyst will receive fixed carbon in the form of carbohydrate from its neighbors through the same channels.
54
Q

Regulation of heterocyst development

A
  • NctA protein senses nitrogen status in the cell by measuring the level of alpha-ketoglutarate/glutamate ratio.
55
Q

ratio of Alpha-ketoglutarate/glutamate rises

A
  • low fixed N

- NctA will induce N2 fixation along with the het genes that will instruct the cell to form a heterocyst.

56
Q

problem of cyanobacteria

A
  • only wants a maximum of 10% of its cells to become heterocyst
57
Q

How cyanobacteria count to 10

A
  • when sensing low N PatS is turned on by NctA
58
Q

PatS

A
  • an inhibitor of differentiation

- the first cell to make significant amounts of PatS will be the one that will form the heterocyst.

59
Q

How PatS works

A
  • most likely binds to and inhibits an important regulatory protein that acts early in heterocyst formation
  • PatS is made and exported by the first cells to sense N-starvation.
  • It diffuses into and through adjacent cells, and a concentration gradient of the protein is set up, cells nearest the heterocyst get the highest dose and it decreases as the cells get further away.
  • About 10 cells away, the level of PatS is low enough that the cell can initiate heterocyst formation.
60
Q

once the cell has differentiated to the point of making PetS

A
  • it is immune to the inhibition.
61
Q

Expression of Nif through recombination

A
  • in vegetative cells, two nitrogenase operons are interrupted by an 11 kb DNA segment bound by 11 bp
  • nonreversible regulation
62
Q

XisA and XisF recombinases

A
  • stimulated by NctA

- removes the insertion sequences and make a functional Nif operon.

MB 414 (26 decks)

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