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In-Depth Information
Finkler et al. (
2007
) found that the Ca
2+
/CaM-responsive CAMTAs bind to the
ABA-responsive ABRE
cis
elements in
Arabidopsis
, suggesting a link between
Ca
2+
-responsive transcription factors and ABA-responsive
cis
-elements. CAMTA3
directly interacts with the promoter of the
EDS1
gene, a regulator of salicylic acid
levels, and represses its expression (Du et al.
2009
). Ca
2+
/calmodulin binding to
CAMTA3 is required for the suppression of plant defense, indicating a direct role of
Ca
2+
/calmodulin in regulating the function of CAMTA3 (Du et al.
2009
).
4.14.11
Calmodulin-Binding OsCBT, NtER1, and AtSR1
Transcription Factors
Some transcription factors of the basic helix-loop-helix family were shown to bind
calmodulin (Onions et al.
2000
). A CaM-binding transcription factor, OsCBT
( O
ryza
s
ativa
CaM-binding transcription factor), has been isolated from rice (Choi
et al.
2005
). It contains a CG-1 homology DNA binding domain, three ankyrin
repeats, a putative transcriptional activation domain, and fi ve putative CaM-binding
motifs (Choi et al.
2005
). OsCBT has two different types of functional CaM-binding
domains, an IQ motif, and a Ca
2+
-dependent motif (Choi et al.
2005
).
Some ethylene-responsive genes-encoded proteins in tobacco, tomato, rapeseed,
Arabidopsis
have structural similarity to the rice CaM-binding transcription factor,
OsCBT (Choi et al.
2005
). The tobacco early ethylene-responsive gene
NtER1
encodes
a CaM-binding protein (Yang and Poovaiah
2000
). One
NtER1
homolog (
AtSR1
) and
fi ve related genes (
AtSR2-6
) have been identifi ed in
Arabidopsis
and they are desig-
nated as
AtSR
(for A
rabidopsis
t
haliana
S
ignal
R
esponsive
) genes (Yang and Poovaiah
2002a
). These six genes exhibit rapid and differential response to signal elicitors such
as ethylene, jasmonate (JA), salicylic acid, ABA, and H
2
O
2
. Ca
2+
/CaM binds to a
23-mer peptide in all AtSRs that corresponds to the CaM-binding region of NtER1.
Each AtSR has a conserved structural feature with a DNA-binding domain in the N
terminus and a CaM-binding domain in the C terminus. AtSR1 targets the nucleus and
specifi cally recognizes a novel 6-bp CGCG box (A/C/G) CGCG (G/T/C). The multi-
ple CGCG
cis
-elements are found in promoters of various genes involved in ET- and
ABA signaling systems. The results suggest that AtSR may interact with ET and ABA
signaling systems (Yang and Poovaiah
2002a
).
4.14.12
Calmodulin-Binding WRKY Transcription Factors
WRKY proteins constitute a large class of transcription factors. More than 70
WRKY genes have been reported in
Arabidopsis
(Eulgem et al.
2000
; Robatzek and
Somssich
2001
; Dong et al.
2003
; Bhattarai et al.
2010
). About 109 WRKY genes
have been identifi ed in rice (Qu and Zhu
2006
; Liu et al.
2005
,
2007
; Qiu et al.
2007
).
The WRKY transcription factor family is defi ned by a domain of 60 amino acids,
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