General Microbiology (MICR300)
Lecture 6
Microbial Physiology
(Text Chapters: 3; 4.14; 4.16 and 8.1-8.7)
Flagella and Motility
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Flagella move the cell by rotation, much
like the propeller in a motor boat (Figure
4.56). An appreciable speed of about 60
cell lengths/second can be achieved.
Flagella are made up of the protein
flagellin and can occur in a variety of
locations and arrangements. Each
arrangement is unique to a particular
species.
Flagella and Motility
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In polar flagellation, the flagella are
attached at one or both ends of the cell.
In peritrichous flagellation, the flagella
are inserted at many locations around the
cell surface (Figure 4.58).
Chemotaxis
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Motile bacteria can respond to chemical
and physical gradients in their
environment.
Chemotaxis (Figure 4.61) is the
directed movement of organisms in
response to chemicals.
Chemotaxis (Continued)
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In chemotaxis, random movement of a
prokaryotic cell can be biased either toward or
away from a stimulus by controlling the degree
to which runs or tumbles occur. The latter are
controlled by the direction of rotation of the
flagellum, which in turn is controlled by a
network of sensory and response proteins.
Counterclockwise rotation moves the cell in a
direction called a run. Clockwise rotation causes
the tuft of flagella to spread, resulting in
tumbling of the cell.
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Attractants and Repellants
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Positive chemotaxis is occurring toward
an attractant when the sum of bacterial
runs, or movement from flagella rotation,
results in net movement in the direction
of increasing concentration of a chemical.
In contrast, motile bacteria will move
away from a repellant (Figure 4.62).
Q: which beaker contains a capillary tube
filled with repellent?
Metabolic Regulation
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There are two major levels of regulation
in the cell.
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One controls the activity of preexisting
enzymes.
One controls the amount of an enzyme
produced.
Regulation of Enzyme Activity
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Enzyme activity can be controlled via:
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Noncovalent Enzyme Inhibition, or
Covalent Modification of Enzymes.
Noncovalent Enzyme Inhibition
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Many metabolic reactions can be
regulated through control of the activities
of the enzymes that catalyze them.
An important type of regulation of
enzyme activity is feedback inhibition
(Figure 8.2), in which the final product
of a biosynthetic pathway inhibits the first
enzyme unique to that pathway.
Allosteric Enzyme
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An allosteric enzyme has two binding
sites, the active site, where the substrate
binds, and the allosteric site, where the
inhibitor (called an effector) binds
reversibly (Figure 8.3).
Isoenzymes
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Some biosynthetic pathways under
feedback inhibition employ isoenzymes,
different proteins that catalyze the same
reaction but are subject to different
regulatory controls (Figure 8.5).
Covalent Modification of
Enzymes
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Covalent modification is a regulatory
mechanism for changing the activity of an
enzyme. Enzymes regulated in this way
can be reversibly modified. One type of
modification is adenylylation (the addition
of AMP) (Figure 8.6).
Gene Expression Regulation
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The amount of enzyme produced can be
controlled at the transcription level - gene
expression.
This can be either positive control or
negative control
Negative Control of Transcription:
Repression and Induction
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Negative control involves prevention of
transcription by regulatory proteins called
repressors.
The amount of an enzyme in the cell can
be controlled by decreasing (repression,
Figures 8.11, 8.13) or increasing
(induction, Figure 8.12) the amount of
mRNA that encodes the enzyme.
Positive Control of Transcription
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Positive control of transcription is
implemented by regulators called activator
proteins. They bind to activator-binding sites
on the DNA and stimulate transcription. As in
repressors, activator protein activity is modified
by effectors.
For positive control of enzyme induction, the
effector promotes the binding of the activator
protein and thus stimulates transcription
(Figures 8.14, 8.15).