Transcript W09micr430Lec16 - Cal State LA

Bacterial Physiology (Micr430)
Lecture 16
Bacterial Development
(Text Chapter: 18.15; 18.18)
Bacterial development
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Cellular differentiation in which a cell acquires
phenotypic properties that clearly differentiate
it from a precursor cell;
Cellular differentiation in which a cell divides to
produce 2 daughter cells that can be
distinguished morphologically and/or
physiologically; or
Multicellular development to form specialized
structures – fruiting bodies and biofilms
Bacterial development
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During development (or differentiation),
differential gene expression requires cellto-cell signaling (quorum sensing) as well
as signaling within cells (two-component
systems)
Differentiation Advantages
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Generating “resting” cell forms that are more
resistant to environmental stresses such as
endospores and myxospores
Generating a number of cell forms specifically
“designed” for dispersal of bacteria (such as
swarmer cells of Caulobacter)
Producing cell forms performing specific
functions (heterocysts of Anabaena)
Producing cell forms designed to establish a
symbiotic relationship with another organism
for their mutual benefit such as the nitrogenfixing nodules.
Endospore Forming Genera
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Six separate genera of bacteria produce a
developmental forms called endospores
Bacillus Endospore Formation
Life Cycle of Bacillus
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All endospore-forming bacteria undergo a life
cycle that includes vegetative growth in the
presence of adequate nutrition and favorable
environmental conditions.
At the beginning of the stationary phase, when
nutrients become limiting, the cells have a
variety of overlapping genetic networks
available with which they can respond to this
changing environment.
Among the processes that can be activated are
motility and transformation competence.
The decision to undergo sporulation is a
response of last resort to survive.
Life Cycle of Bacillus
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As cells enter stationary phase, nutritional
deprivation can trigger entry into sporulation.
This process can be visualized by light and
electron microscopy as a series of complex
morphological changes that result in the
formation of a highly resistant dormant form
called endospore.
When conditions become favorable again, the
spore can undergo activation, germination and
outgrowth into a metabolically active cell
capable of entering vegetative growth cycle.
Sporulation in Bacillus
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The decision to sporulate is regulated by
a phosphorelay signal transduction
system.
Sporulation is only one of several
physiological changes during adaptation
to nutrient deprivation.
Motility increases chances of finding
nutrients
Motile cells secrete degradative enzymes
to generate food sources.
Stages of Sporulation
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Sporulation can be divided into seven
stages.
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Stage
Stage
Stage
Stage
Stage
Stage
Stage
Stage
0 (vegetative cells)
I (axial filament formation)
II (septum and prespore formation)
III (forespore development)
IV (cortex formation)
V (coat formation)
VI (maturation)
VII (release of the mature spore)
Stages of Sporulation in Bacillus
Sporulation in Bacillus
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Sporulation is an example of cell division
ending in 2 different developmental fates
for the daughter cells.
B. subtilis differentially expresses genes
in the mother cell and forespore,
resulting in two different types of cells,
by compartmentalization of sigma
factors, which determine which genes
are expressed.
Sporulation genes
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B. subtilis sporulates when it expresses spo
genes, which were discovered by
examining mutants that failed to complete
sporulation.
The genes are named after the stage of
blockage and are distinguished from one
another by a letter.
E.g., mutants in spo0A fail to initiate
sporulation and do not proceed to stage I
Sigma Factors in Bacillus subtilis
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Sigma subunit of RNAP determines the
specificity of promoter utilization.
There are at least 10 different sigma
factors in B. subtilis, each of which
directs RNAP to a different set of
promoters.
Most sigma factors make sequencespecific contacts at -10 and -35 regions
upstream of regulated genes or operons.
Spo0A Protein
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The amino-terminal portion of the Spo0A
protein is homologous to response regulators of
procaryotic two-component systems. It is this
portion of the molecule that is phosphorylated
(via phosphorelay) to activate its transcription
functions.
The carboxy half of the protein contains the
DNA-binding specificity of the protein and
interacts with the transcriptional machinery.
Higher levels of Spo0A-P stimulate axial
filament formation, polar septation and
transcription of genes required for cell typespecific gene expression.
Phosphorelay
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There are five histidine sensor kinases involved in
sporulation but two are most important (Kinases A and
B) for sporulation in lab media by phosphorylating
Spo0F.
Spo0F has a strong similarity to the response regulators
in two-component systems except it lacks an additional
carboxy-terminal domain. Its function is to accept
phosphate from the activating kinases for the
phosphorelay and serve as a substrate for the Spo0B
protein.
Spo0B protein is the phosphoprotein phosphotransferase. The enzymatic function of the Spo0B is to
transfer phosphate from Spo0F-P to Spo0A, producing
Spo0A-P.
Phosphorelay
system
Role of phosphatases
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In phosphorelay that controls sporulation,
the levels of the phosphorylated response
regulator proteins are determined by
multiple specialized phosphatases
The existence of multiple phosphatases
alllows multiple inputs into the regulation
of the phosphorelay pathway
One of the phosphatases, Spo0E,
dephosphorylates the master
transcriptional factor Spo0A-P
Role of phosphatases
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Two of the phosphatases, RapA and
RapB, dephosphorylate the phosphorelay
protein, Spo0F-P
A two-component signal system
consisting of ComP (HK) and ComA (RR)
regulates gene expression of an operon
containing rapA and phrA genes
Since ComP/ComA system produces an
inhibitor of the RapA phosphatase, it
stimulates sporulation
Regulation of RapA
phosphatase