Calcium Ion Signaling System: Calcium Signatures and Sensors - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 235

MYB transcription factors contain one or more MYB domains (Stracke et al. 2001 ).
MYB proteins are classifi ed into subfamilies depending on the number of conserved
repeats of the MYB domain they contain (Mengiste et al. 2003 ). A common feature
of MYB proteins is the presence of a functional DNA binding domain that typically
consists of one to three repeats of the MYB domain. The three MYB repeats are
referred to as R1, R2, and R3. Each repeat is about 50-53 amino acids long and
encodes three -helices, with the second and third helices forming a helix-turn-helix

structure which intercalates in the major groove of DNA when bound to it. MYB
repeats typically contain regularly spaced tryptophan residues, which build a central
tryptophan cluster in the three-dimensional helix-turn-helix fold (Du et al. 2009 ).
MYB transcription factors bind to the cis -regulatory element such as MYB boxes
(Laquitaine et al. 2006 ).
The R2R3-MYB subfamily of transcription factors is the most common in
plants (Stracke et al. 2001 ). A soybean CaM, Gm-Cam4, has been reported to
mediate Ca 2+ signaling response by activating an R2R3-MYB2 transcription fac-
tor (Yoo et al. 2005 ). A grapevine R2R3-MYB transcription factor gene, VvMYB5a ,
induces the expression of genes controlling the biosynthesis of defense-related
phenylpropanoids (Deluc et al. 2006 ). The Arabidopsis BOS1 ( BOTRYTIS
SUSCEPTIBLE 1 ) gene, which encodes a R2R3-MYB transcription factor,
induces disease resistance against P. syringae and disruption of the gene enhances
disease symptom development after infection by P. syringae (Mengiste et al.
2003 ). A R2R3-MYB-like transcription factor, MYB72, has been shown to be
involved in triggering defense responses against broad-spectrum of pathogens in
Arabidopsis thaliana (Van der Ent et al. 2008 ). Arabidopsis MYB2 has been
reported to function as transcriptional activator in abscisic acid signaling (Abe
et al. 2003 ). Several members of the MYB class of transcription factors have been
shown to bind Ca 2+ /CaM (Popescu et al. 2007 ). Direct interaction of a CaM iso-
form with the transcription factor MYB2 has been reported in Arabidopsis
(Yoo et al. 2005 ).

α

4.14.14 Calmodulin-Binding TGA Transcription Factors

The basic leucine zipper (bZIP) proteins belong to a large family of transcription
factors. The bZIP family transcription factors contain a basic region for binding
DNA and a leucine zipper dimerization domain (Jakoby et al. 2002 ). These proteins
have a conserved region rich in basic amino acid residues that binds to the target
DNA and contains nuclear localization signals (NLSs) and dimerization or multi-
merization domains. A leucine zipper region, which consists of several heptad
repeats of hydrophobic residues, is found close to the basic region. The leucine
zipper region is alpha-helical and prone to dimer formation via a coiled-coil arrange-
ment (Kuhlmann et al. 2003 ; Meng et al. 2005 ).
The TGA class of transcription factors is the important group of bZIP transcrip-
tion factors involved in defense signaling. Members of the TGA family of