Gene Expression and Signal Transduction

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Chapter 14. Gene Expression and Signal
Transduction
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Signal transduction in Prokaryotes
Bacteria Employ Two-Component Regulatory Systems
to Sense Extracellular Signals
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Osmolarity Is Detected by a Two-Component System
Figure 14.12 E. coli two-component system for
osmo-regulation.
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Related Two-Component Systems Have Been
Identified in Eukaryotes
SLN1 gene of yeast encodes a 134 kd protein that
has transmitter and receiver domains and appears
to function in osmo-regulation
Plant signaling systems evloved from bacterial
two component systems

• the red/far-red absorbing pigment, phytochrome
• 2. putative cytokinin receptors
• 3. putative ethylene receptors

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Signal transduction in Eukaryotes
Two Classes of Signals Define Two Classes of
Receptors
- Lipophilic hormones bind mainly to receptors in
the cytoplasm or nucleus - water-soluble
hormones bind to receptors located on the cell
surface.
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Second messengers in plant
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Most Steroid Receptors Act as Transcription
Factors
Figure 14.14. Glucocorticoid steroid receptors
are transcription factors.
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Steroid hormone in plants
- Brassinosteroid has recently been demonstrated
to be an authentic steroid hormone in plants. -
The gene for a brassinosteroid receptor has
recently been cloned and sequenced. -
Brassinosteroid receptor encodes a type of
transmembrane receptor called a leucine-rich
repeat receptor.
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Cell Surface Receptors Can Interact with G
Proteins (GPCR G-protein coupled
receptor)
Figure 14.15 Schematic drawing of two types of
seven transmembrane receptors.
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Heterotrimeric G Proteins Cycle between Active
and Inactive Forms
Figure 14.16 Hormone-induced activation of an
effector enzyme is mediated by the a subunit of a
heterotrimeric G protein.
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cAMP
Opens Na Channel
Membrane depolarization
PKA
CREB
P
CREB
CRE

• cAMP mediated signaling pathway in plant
• Homologs of CREB have been identified
• Pollen tube growth has been to be stimulated by
• cAMP activate K channels
• 3. Identified possible genes for AC

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Activation of Phospholipase C Initiates the IP3
Pathway
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IP3 Opens Calcium Channels on the ER and on the
Tonoplast
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Cyclic ADP-Ribose Mediates Intracellular Ca2
Release Independently of IP3 Signaling

• Cyclic ADP-Ribose (cADPR) acts as a second
  messenger that can release
• calcium from intracellular stores, independent
  of the IP3 signaling pathway.
• - Like cAMP, cADPR is a cyclic nucleotide, and
  cADPR binds to and activates
• specific calcium channels, called type-3
  ryanodine receptors
• (ryanodine is a calcium channel blocker).
• These ryanodine receptor/calcium channels are
  located on the membranes
• of calcium-storing organelles, such as SR of
  animal cells or the vacuoles of
• plant cells.

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Some Protein Kinases Are Activated by
CalciumCalmodulin Complexes (CaM
Kinases)
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Calmodulin
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Plants Contain Calcium-Dependent Protein Kinases
(CDPKs)
The most abundant calcium in plants appear to be
the calcium-dependent protein kinases (CDPKs).
CDPKs are strongly activated by calcium, but
are insensitive to calmodulin. The proteins are
characterized by two domains a catalytic domain
that is similar to those of the animal CaM
kinases, and a calmodulin-like domain. CDPKs
mediate the effects of calcium by phosphorylating
the heavy chain of myosin, a component of the
microfilaments. CDPKs may also mediate the
effects of calcium in guard cells. Abscisic
acidinduced stomatal closure involves calcium as
a second messenger.
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Cellular Function of CDPK
Regulation of CDPK
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Diacylglycerol Activates Protein Kinase C
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Phospholipase A2 Generates Other Membrane-Derived
Signaling Agents
Figure 14.20. Eicosanoid biosynthetic pathway.
(A) The first step is the hydrolysis of
20-carbon fatty acid chains containing at least
three double bonds from a membrane phospholipid
by the enzyme phospholipase A2, producing
arachidonic acid, which can be oxidized by
prostaglandin. (B) Arachidonic acid is further
metabolized by two pathways one cyclooxygenase
dependent, the other lipoxygenase dependent.
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Eicosanoid pathway in plant Jasmonic acid (JA)
is produced by a similar pathway JA is an
important signaling agent in plant defenses
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In Vertebrate Vision, a Heterotrimeric G Protein
Activates Cyclic GMP Phosphodiesterase
(Ga)
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Figure 14.22 The role of cyclic GMP (cGMP) and
calcium as second messengers in vertebrate vision.
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• cGMP in plants
• cGMP appears to be an important regulatory
  molecules
• in plant cells
• 2. cGMP has been implicated as a 2nd messenger
• in response of phytochrome and GA

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Nitric Oxide Gas Stimulates the Synthesis of
cGMP In smooth muscle tissue of animal cells,
cGMP levels can be increased via the direct
activation of guanylyl cyclase by nitric oxide
(NO). NO is synthesized from arginine by the
enzyme, NO synthase, in a reaction involving
oxygen NO synthase Arginine O2?Citrulline
NO Once produced in animal endothelial cells,
dissolved NO passes rapidly across membranes and
acts locally on neighboring smooth muscle cells,
with a half-life of 510 seconds. Guanylyl
cyclase contains a heme group that binds NO
tightly, and binding of NO causes a
conformational change which activates the enzyme.
The NO-induced increase in cGMP causes smooth
muscle cells to relax. In plants, NO has
recently been implicated as an intermediate in
ABA-induced stomatal closure.