Chapter 16
Prokaryotic cell biology
By
Jeff Errington, Matthew Chapman, Scott J.
Hultgren, & Michael Caparon
16.1 Introduction
• The relative simplicity of the prokaryotic
cell architecture compared with
eukaryotic cells belies an economical
but highly sophisticated organization.
16.1
Introduction
• A few prokaryotic species are well
described in terms of cell biology.
– These represent only a tiny sample of the
enormous diversity represented by the
group as a whole.
• Many central features of prokaryotic cell
organization are well conserved.
16.1
Introduction
• Diversity and adaptability have been
facilitated by a wide range of optional
structures and processes.
– These provide some prokaryotes with the ability to
thrive in specialized and sometimes harsh
environments.
• Prokaryotic genomes are highly flexible.
• A number of mechanisms enable prokaryotes
to adapt and evolve rapidly.
16.2 Molecular phylogeny techniques are
used to understand microbial evolution
• Only a fraction of the prokaryotic
species on Earth has been analyzed.
16.2 Molecular phylogeny techniques are used to understand microbial
evolution
• Unique taxonomic techniques have
been developed for classifying
prokaryotes.
• Ribosomal RNA (rRNA) comparison has
been used to build a three-domain tree
of life that consists of:
– Bacteria
– Archaea
– Eukarya
16.3 Prokaryotic lifestyles are diverse
• The inability to culture many prokaryotic
organisms in the laboratory has
hindered our knowledge about the true
diversity of prokaryotic lifestyles.
16.3 Prokaryotic lifestyles are
diverse
• DNA sampling has been used to better gauge
the diversity of microbial life in different
ecological niches.
• Prokaryotic species can be characterized by
their ability to survive and replicate in
environments that vary widely in:
–
–
–
–
temperature
pH
osmotic pressure
oxygen availability
16.4 Archaea are prokaryotes with
similarities to eukaryotic cells
• Archaea tend to:
– be adapted to life in extreme environments
– utilize “unusual” energy sources
• Archaea:
– have unique cell envelope components
– lack peptidoglycan cell walls
16.4 Archaea are prokaryotes with similarities to eukaryotic
cells
• Archaea resemble bacteria in:
– their central metabolic processes
– certain structures, such as flagella
• Archaea resemble eukaryotes in terms of:
– DNA replication
– Transcription
– Translation
• However, gene regulation involves many
Bacteria-like regulatory proteins
16.5 Most prokaryotes produce a
polysaccharide-rich layer called the
capsule
• The outer surface of many prokaryotes
consists of a polysaccharide-rich layer called
the capsule or slime layer.
• The proposed functions of the capsule or
slime layer are:
– to protect bacteria from desiccation
– to bind to host cell receptors during colonization
– to help bacteria evade the host immune system
16.5 Most prokaryotes produce a polysaccharide-rich layer called the
capsule
• E. coli capsule formation occurs by one
of at least four different pathways.
• In addition to, or in place of the capsule,
many prokaryotes have an S-layer.
– This is an outer proteinaceous coat with
crystalline properties.
16.6 The bacterial cell wall contains a
crosslinked meshwork of peptidoglycan
• Most bacteria have peptidoglycan:
– a tough external cell wall made of a polymeric
meshwork of glycan strands crosslinked with short
peptides.
• The disaccharide pentapeptide precursors of
peptidoglycan are:
– synthesized in the cytoplasm
– Exported
– assembled outside the cytoplasmic membrane
16.6 The bacterial cell wall contains a crosslinked meshwork of
peptidoglycan
• One model for cell wall synthesis is that
a multiprotein complex carries out
insertion of new wall material following a
“make-before-break” strategy.
• Many autolytic enzymes remodel,
modify, and repair the cell wall.
16.6 The bacterial cell wall contains a crosslinked meshwork of
peptidoglycan
• For some bacteria, the peptidoglycan
cell wall is important for maintaining cell
shape.
• A bacterial actin homolog, MreB, forms
helical filaments in the cell cytoplasm.
– They direct the shape of the cell through
control of peptidoglycan synthesis.
16.7 The cell envelope of Gram-positive
bacteria has unique features
• Gram-positive bacteria have a thick cell
wall containing multiple layers of
peptidoglycan.
• Teichoic acids are an essential part of
the Grampositive cell wall.
– Their precise function is poorly understood.
16.7 The cell envelope of Gram-positive bacteria has unique
features
• Many Gram-positive cell surface
proteins are covalently attached to:
– membrane lipids or
– peptidoglycan
• Mycobacteria have specialized lipid-rich
cell envelope components.
16.8 Gram-negative bacteria have an
outer membrane and a periplasmic space
• The periplasmic space is found between
the cytoplasmic and outer membranes
in Gram-negative bacteria.
16.8 Gram-negative bacteria have an outer membrane and a
periplasmic space
• Proteins destined for secretion across
the outer membrane often interact with
molecular chaperones in the
periplasmic space.
• The outer membrane is a lipid bilayer
that prevents the free dispersal of most
molecules.
16.8 Gram-negative bacteria have an outer membrane and a
periplasmic space
• Lipopolysaccharide is a component of
the outer leaflet of the outer membrane.
• During infection by Gram-negative
bacteria, lipopolysaccharide activates
inflammatory responses.
16.9 The cytoplasmic membrane is a
selective barrier for secretion
• Molecules can pass the cytoplasmic
membrane by:
– passive diffusion
– active translocation
16.9 The cytoplasmic membrane is a selective barrier for
secretion
• Specialized transmembrane transport
proteins mediate the movement of most
solutes across membranes.
• The cytoplasmic membrane maintains a
proton motive force between the
cytoplasm and the extracellular milieu.
16.10 Prokaryotes have several secretion
pathways
• Gram-negative and Gram-positive
species use the Sec and Tat pathways
for transporting proteins across the
cytoplasmic membrane.
16.10 Prokaryotes have several secretion
pathways
• Gram-negative bacteria also transport
proteins across the outer membrane.
• Pathogens have specialized secretion
systems for secreting virulence factors.
16.11 Pili and flagella are appendages on
the cell surface of most prokaryotes
• Pili are extracellular proteinaceous
structures that mediate many diverse
functions, including:
– DNA exchange
– adhesion
– biofilm formation by prokaryotes
16.11 Pili and flagella are appendages on the cell surface of most
prokaryotes
• Many adhesive pili are assembled by
the chaperone/usher pathway, which
features:
– an outer membrane
– usher proteins that form a pore through
which subunits are secreted
– a periplasmic chaperone that:
• helps to fold pilus subunits
• guides pilus subunits to the usher
16.11 Pili and flagella are appendages on the cell surface of most
prokaryotes
• Flagella are extracellular apparati that
are propellers for motility.
• Prokaryotic flagella consist of multiple
segments.
– Each is formed by a unique assembly of
protein subunits.
16.12 Prokaryotic genomes contain
chromosomes and mobile DNA elements
• Most prokaryotes have a single circular
chromosome.
• Genetic flexibility and adaptability is
enhanced by:
– transmissible plasmids
– bacteriophages
• Transposons and other mobile elements
promote the rapid evolution of prokaryotic
genomes.
16.13 The bacterial nucleoid and
cytoplasm are highly ordered
• The bacterial nucleoid appears as a
diffuse mass of DNA but is highly
organized.
– Genes have nonrandom positions in the
cell.
• Bacteria have no nucleosomes.
– A variety of abundant nucleoid-associated
proteins may help to organize the DNA.
16.13 The bacterial nucleoid and cytoplasm are highly
ordered
• In bacteria, transcription takes place
within the nucleoid mass.
• Translation takes place within the
peripheral zone.
– Analogous to the nucleus and cytoplasm of
eukaryotic cells
• RNA polymerase may make an
important contribution to nucleoid
organization.
16.14 Bacterial chromosomes are
replicated in specialized replication
factories
• Initiation of DNA replication is a key
control point in the bacterial cell cycle.
• Replication takes place bidirectionally
from a fixed site called oriC.
16.14 Bacterial chromosomes are replicated in specialized replication
factories
• Replication is organized in specialized
“factories.”
• Replication restart proteins facilitate the
progress of forks from origin to
terminus.
• Circular chromosomes usually have a
termination trap.
– This ensures that replication forks
converge in the replication terminus region.
16.14 Bacterial chromosomes are replicated in specialized replication
factories
• Circular chromosomes require special
mechanisms to coordinate termination with:
–
–
–
–
decatenation
dimer resolution
segregation
cell division
• The SpoIIIE (FtsK) protein completes the
chromosome segregation process by
transporting any trapped segments of DNA
out of the closing division septum.
16.15 Prokaryotic chromosome
segregation occurs in the absence of a
mitotic spindle
• Prokaryotic cells have no mitotic
spindle, but they segregate their
chromosomes accurately.
• Measurements of oriC positions on the
chromosome show that they are actively
separated toward opposite poles of the
cell early in the DNA replication cycle.
16.15 Prokaryotic chromosome segregation occurs in the absence of a mitotic
spindle
• The mechanisms of chromosome
segregation are poorly understood.
– Probably because they are partially
redundant
• The ParA-ParB system is probably
involved in chromosome segregation in
many bacteria and low-copy-number
plasmids.
16.16 Prokaryotic cell division involves
formation of a complex cytokinetic ring
• At the last stage of cell division, the cell
envelope undergoes either:
– constriction and scission, or
– septum synthesis followed by autolysis
…to form two separate cells.
• A tubulin homolog, FtsZ, orchestrates the
division process in bacteria, forming a ring
structure at the division site.
16.16 Prokaryotic cell division involves formation of a complex cytokinetic
ring
• A set of about 8 other essential division
proteins assemble at the division site
with FtsZ.
• The cell division site is determined by
two negative regulatory systems:
– nucleoid occlusion
– the Min system
16.17 Prokaryotes respond to stress with
complex developmental changes
• Prokaryotes respond to stress, such as
starvation, with a wide range of adaptive
changes.
16.17 Prokaryotes respond to stress with complex developmental
changes
• The simplest adaptative responses to
stress involve:
– changes in gene expression and
metabolism
– a general slowing of the cell cycle,
preparing the cell for a period of starvation
• In some cases, starvation induces
formation of highly differentiated
specialized cell types.
– For example, the endospores of Bacillus
subtilis.
16.17 Prokaryotes respond to stress with complex developmental
changes
• During starvation, mycelial organisms
such as actinomycetes have complex
colony morphology and produce:
– aerial hyphae
– spores
– secondary metabolites
• Myxococcus xanthus exemplifies
multicellular cooperation and
development of a bacterium.
16.18 Some prokaryotic life cycles
include obligatory developmental
changes
• Many bacteria have been studied as
simple and tractable examples of
cellular development and differentiation.
• Caulobacter crescentus is an example
of an organism that produces
specialized cell types at every cell
division.
16.19 Some prokaryotes and eukaryotes
have endosymbiotic relationships
• Mitochondria and chloroplasts arose by
the integration of free-living prokaryotes
into the cytoplasm of eukaryotic cells.
– There, they became permanent symbiotic
residents.
16.19 Some prokaryotes and eukaryotes have endosymbiotic
relationships
• Rhizobia species form nodules on
legumes:
– So that elemental nitrogen can be
converted into the biologically active form
of ammonia.
• The development and survival of pea
aphids depends on an endosymbiotic
event with Buchnera bacteria.
16.20 Prokaryotes can colonize and
cause disease in higher organisms
• Although many microbes make their homes in
or on the human body, only a small fraction
cause harm to us.
• Pathogens are often able to:
– colonize
– replicate
– survive within host tissues
• Many pathogens produce toxic substances to
facilitate host cell damage.
16.21 Biofilms are highly organized
communities of microbes
• It has been estimated that most of the
Earth’s prokaryotes live in organized
communities called biofilms.
16.21 Biofilms are highly organized communities of
microbes
• Biofilm formation involves several steps
including:
– surface binding
– growth and division
– polysaccharide production
– biofilm maturation
– dispersal
• Organisms within a biofilm
communicate by quorum sensing
systems.