532 XIANG
genes may have one or more sulfhydryl groups exposed on the surface. The nucleophilic SH groups would
be apparent targets for the Michael acceptor of these compounds in a way similar to the conjugation to
GSH. The conjugation of these compounds to the signaling components could somehow activate the sig-
naling pathways leading to strong induction of GST gene expression.
The activation of as-1–type elements in response to stress signal molecules, H 2 O 2 , jasmonic acid,
and salicylic acid, is also well demonstrated [25,26,31,40,43,47]. A wealth of information has been ac-
cumulated for the responses of defense genes to these stress signal molecules that are the subjects of a
number of reviews [13,48,49]. H 2 O 2 and salicylic acid are endogenous signal molecules that are elicited
upon pathogen infection [13], and jasmonic acid production is triggered upon wounding and insect feed-
ing [35,50]. In addition, the production of these signal molecules may be elicited and accelerated by abi-
otic stress conditions. Cellular H 2 O 2 concentration can be increased on exposure to heavy metals and ox-
idative stress conditions.
However, variations of the responsiveness to the stress signals do exist from promoter to promoter
of defense genes. The different responsiveness may result from the variation in the cognate as-1–type el-
ements in different defense gene promoters. Of course, other ciselements in the promoter context also
contribute to the expression. It is the combinatorial effect of all the ciselements that determines a pro-
moter activity [51].
V. THE as-1–TYPE ELEMENTS SPECIFICALLY INTERACT WITH A
SUBCLASS OF BZIP TRANSCRIPTION FACTORS CALLED TGA
FACTORSThe bZIP proteins are transcription factors that contain a basic region for specific DNA contact and a
leucine zipper domain for dimerization. All bZIP factors bind to specific DNA sequences as homo- or het-
erodimers [52]. These transcription factors are believed to contribute to the efficiency with which RNA
polymerase II binds and initiates transcription at the promoter of a gene. They are generally activators of
transcription in response to external stimuli either constitutively or in a regulated manner, usually through
posttranslational modifications such as phosphorylation. However, bZIP proteins can also be repressors
in some cases [53,54]. Many bZIP factors are also expressed in a cell type–specific or developmentally
regulated fashion.
In vitro studies using gel retardation assays have firmly established the specific interaction between
as-1–type elements and a subclass of bZIP transcription factors called TGAs in plants [1,2,55,56]. Lam
and Lam [57], with supershift assays using specific antisera raised against specific TGA factors,
demonstrated that TGA factors are integral components of the in vivo as-1element–binding activity
called ASF-1.
The TGA factors are highly conserved in higher plants. A small gene family exists for the TGA bZIP
transcription factors in Arabidopsis. Six members of the ArabidopsisTGA family of bZIP transcription
factors have been cloned and characterized. These genes were designated as TGA1[58],TGA2[59]TGA3
[32],OBF4andOBF5[60] and TGA6[61]. Based on sequence homology, TGA family members may be
divided into three subgroups: (1) TGA2, OBF5, and TGA6; (2) TGA1andOBF4; and (3) TGA3, which
shows less than 66.2% homology in DNA sequence to the other known TGA family members [61]. The
seventh member of this gene family, TGA7, was recently isolated [62].
The multiple gene family of TGA factors provides functional redundancy and subtle variations that
may be required for fine-tuning the regulation of target genes. Theoretically, these seven bZIP factors are
able to produce 28 different types of dimers. These dimer combinations could generate further diversity
in the types of transcriptional regulation mediated by the TGA factors. Distinct DNA binding properties
have been shown in vitro for different TGA factors, although the basic region for DNA contact is well
conserved among different members of the TGA family [57,61]. If the same is true in vivo, preferential
binding could introduce additional subtlety in regulation, which may be required in selectively activating
or repressing target genes through interaction with a particular ciselement in the context of the target gene
promoters in response to different environmental stress conditions. Besides, variant TGA factor binding
sites are indeed present in the promoters of defense-related genes [20,45].