Transcript Procaryotic structure and function

Prokaryotic Cell Structure
and function (Part II)
BIO3124
Lecture #3 (II)
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Flagella and Motility
• 15-20 um long appendages
• extended through CW and anchored to CM
• Provide motility
• differently distributed on each cell type
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Patterns of Flagella Distribution

monotrichous – one flagellum
usually polar (ie. flagellum at one
end of cell)

amphitrichous – one flagellum at
each end of cell

lophotrichous (tuft): cluster of
flagella at one or both ends

peritrichous – spread over entire
surface of cell
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Flagellar Ultrastructure
Gram negative
Gram positive
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Animation: Bacterial flagellum rotation mechanism
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The filament

extends from cell surface to the tip

hollow, rigid cylinder

composed of the protein flagellin

some procaryotes have a sheath around filament, eg.
Spirochetes
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Flagellum Synthesis


an example of
self-assembly
complex
process
involving many
genes and
gene products

new molecules
of flagellin are
transported
through the
hollow filament

growth is from
tip, not base
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Chemotaxis
Chemotaxis is the movement of a bacterium in
response to chemical gradients.
Attractants cause CCW rotation.
- Flagella bundle together.
- Push cell forward
- “Run”
Repellents cause CW rotation.
- Flagellar bundle falls apart.
- “Tumble” = Bacterium briefly
stops, then changes direction
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Chemotaxis
The alternating runs and tumbles cause a
“random walk.”
- Receptors detect attractant concentrations.
- Sugars, amino acids
- Attractant concentration increases and
prolongs run.
- This is termed a “biased random walk.”
- Causes a net movement of bacteria
toward attractants (or away from
repellents)
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CCW and CW rotation of flagella
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Chemotaxis: molecular events
Regulated by two-component signaling
Major proteins
 MCPs: Methyl-accepting chemotaxis proteins

clustered at cell poles bind chemoattractants, receptor
sensor and kinase (CheA/CheW), phosphorayte CheY
CheY-P, a response regulator, increase the tumble
frequency
Other regulatory proteins
 CheR & CheB: reversible methylation or demethylation
of MCPs desensitizes or sensitizes MCPs
 CheZ, dephosphorylation of CheY-P
-


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Chemotaxis
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The Nucleoid


Single loop of doublestranded DNA
Attached to cell envelope
 No
membrane separates
DNA from cytoplasm


Replicates once for each
cell division
Compacted via
supercoiling by
topoisomerases I and II
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Plasmids


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

usually small, closed circular DNA molecules
exist and replicate independently of
chromosome
have relatively few genes present
genes on plasmids are not essential to host but
may confer selective advantage (e.g., drug
resistance)
classification of plasmids based on mode of
existence and function eg. R-factors, F-plasmids
and metabolic plasmids
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Cell Division



Cell division, or cell fission, requires highly
coordinated growth and expansion of all the
cell’s parts.
Unlike eukaryotes, prokaryotes synthesize RNA
and proteins continually while the cell’s DNA
undergoes replication.
Bacterial DNA replication is coordinated with the
cell wall expansion and ultimately the separation
of the two daughter cells.
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DNA Replication
In prokaryotes, a circular chromosome
begins to replicate at its origin, or ori site.
 Two replication forks are generated, which
proceed outward in both directions.

- At each fork, DNA is synthesized by DNA
polymerase with the help of accessory
proteins (the replisome).

As the termination site is replicated, the
two forks separate from the DNA.
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Cell Division (Fission)

Cell elongates as it grows
 Adds

new wall at cell equator
DNA replicates to make 2
chromosomes
 DNA
replicates bidirectionally
 Can begin next replication
before cell divides

Cell undergoes septation
 Usually
at equator
 Each daughter has same shape
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Cell Division (Binary Fission)
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Coordination of leading and lagging strands
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Cytokinesis: Role of Cytoskeletal Proteins



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process not well understood
protein MreB
 similar to eucaryotic actin
 determination of cell shape and
movement of chromosomes to
opposite cell poles
protein FtsZ,
 similar to eucaryotic tubulin
 Z ring formation
MinCD protein



inhibitor of FtsZ multimerization
Oscillates between cell poles
Localizes the Z ring to the equatorial
plane
FtsZ
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Divisome
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Cytokinesis &
chromosome replication
coordinately regulated
Fts proteins form
divisome
FtsA, ZipA: anchor Z ring
to cytoplasmic
membrane
FtsK: coordinates
septation with
chromosome partitioning
Others: FtsI,L,N,Q,B,W and
AmiC involved in PG
synthesis
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Gene Expression


RNA Polymerase transcribes DNA to mRNA
Ribosome translates RNA to Protein
 Processes
occur simultaneously
- This is aided by
the signal
recognition particle
(SRP), which binds
to the growing
peptide.
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Special strutures

Cyanobacteria have thylakoids
 Extensively
folded inner membrane
 Contain chlorophyll
 Ancestors of chloroplasts

Carboxysomes fix carbon
 Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase),use
energy to make sugar

Other bacterial photosynthetic pigments
 Purple membranes
Halobacteria
 Phycobilisome
containing Bacteriorhodopsin among
proteins collect light energy
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Organic inclusion bodies

Intracellular deposits of material
 Glycogen
(sugar) for energy
 Parahydroxy butyrate (PHB), fatty acid
polymer for energy
Carboxysomes,
lipid energy-storage granules
 Gas vacuoles
 found in cyanobacteria and some other
aquatic procaryotes, provide buoyancy
 aggregates of hollow cylindrical structures
called gas vesicles
Function: floatation to regulate O2 and light
intensity
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Inorganic inclusion bodies

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Polyphosphate granules
 also called volutin granules or
metachromatic granules
 linear polymers of phosphates, stored
and used in DNA synthesis
sulfur granules: periplasmic or
cytoplasmic, accumulated by sulfur
bacteria
Magnetosomes
contain iron in the form of
magnetite (Fe3O4)
used to orient
magnetotactic bacteria in
magnetic fields
Reviews: Arash, Schuler
Iridescent sulfur granules
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