MECHANISMS OF BZIP ON TRANSCRIPTIONAL REGULATION OF TARGET
GENES
Through
dimerization, phosphorylation, or interaction with other nuclear
proteins, the specificity and affinity of bZIP binding to DNA will
change, which will affect the activation of other genes, as well as its
own stability and subcellular localization (Schütze et al., 2008). By
forming homo- or heterodimers, binding specific gene promoters in its
basic region and interacting with cis -acting elements of the
promoter region, the bZIP transcription factor inhibits or activates the
expression of multiple downstream related target genes, thereby
participating in transcriptional regulation process.
The binding specificity of bZIP
factors in plants is mainly determined by 3 bases flanking the 4 core
nucleotides. Generally, bZIP factors preferentially select ACGT core
palindromes or pseudo-palindromic cis -acting elements to bind,
such as G-box (CACGTG), C-box (GACGTC), A-box (TACGTA), ABRE (ACGTGGC)
(Izawa et al., 1993; Kim et al. 2004). Most of them are located in the
ABA hormone-induced promoter region. When the bZIP protein interacts
with these cis -acting elements, the N-terminus of its basic
domain is inserted into the large groove of the DNA double-strand, and
the C-terminus of the leucine zipper is dimerized to form a superimposed
curl helix (Landschulz et al., 1988; Ellenberger et al., 1992).
G-box is one of the most common
targets of bZIP transcription factors. de Vetten and Ferl (1995) firstly
found that corn GBF1 is a basic region leucine zipper protein and could
activate Adhl expression by binding to its G-box. After that,
series of stress related genes were found to be bound at their G-box and
regulated by various bZIPs. Kaminaka et al. (2006) found thatArabidopsis thaliana AtbZIP10 can combine G-box to
negatively regulate plant resistance to pathogenic bacteria and other
stresses. Zou et al. (2008) demonstrated that the
rice OsbZIP10/OsABI5 could bind
the G-box element to trans -activate the expression of stress
resistance genes, thereby inhibiting seed germination and seedling
growth. Liu et al. (2012) also studied rice and found that under low
temperature stress,OsbZIP52 /RISBZ5 can
recognize the G-box on the downstream target gene to enhance the low
temperature sensitivity of rice. The Arabidopsis thalianaAtbZIP56/HY5 binds directly to the promoters of light responsible
element containing the G-box and thus regulates their transcriptional
activity (Yoon et al., 2006). Induced by salt, the Tamarix
hispida bZIP1 bound to G-box of the stress response gene and regulated
the expression of downstream genes (Ji et al., 2013). Using chromatin
immunoprecipitation, Lee et al. (2006a) demonstrated that CabZIP1 binds
to the G-box elements in native promoter of the hot pepper
pathogenesis-related protein 1 (CaPR-1 ) gene in vivo. Shaikhali
et al. (2012) identified the AtbZIP16 as a component binding to the
G-box-containing promoter fragment of light-harvesting chlorophyll
a/b-binding protein2.4 (LHCB2.4) from nuclear extracts of high
light-treated Arabidopsis plants.
The ABRE element is also the most
favorite target of bZIP transcription factors. Sun et al. (2011) found
that AtbZIP1 binds to ABRE active elements and regulates the plant’s
response to low temperature stress through ABA-dependent signaling
pathways. Yoshida et al. (2015) demonstrated that the Arabidopsis
thaliana bZIP transcription factors ABF1, ABF2, ABF3, and ABF4 combined
with ABRE and regulated the expression of downstream genes related to
salt and drought tolerance. In maize, ZmbZIP17 functions as an ER
stress transducer, interacting with ABREs (Yang et al. 2013). Rice
OsbZIP46/OsABF2 (Chang et al., 2017; Hossain et al., 2010; Tang et al.
2012a), OsbZIP52/RISBZ5 (Liu et al., 2012),OsbZIP10/OsABI5 (Zou et al., 2007; Zou et al., 2008),
OsbZIP05/OSBZ8 (Mukherjee et al., 2006; Nakagawa et al., 1996) all
regulate the expression of plant ABA-responsive genes by binding to the
ABRE element of the target gene. Zhang et al. (2017b) proved that wheat
TabZIP14-B showed transcriptional activation ability through the
transactivation assay and was capable of binding the abscisic acid (ABA)
responsive element (ABRE) in yeast. Wang et al. (2019) isolated and
functionally characterized the sweet potato IbABF4 gene, which
encodes a bZIP transcription factor. The IbABF4 protein localized to the
nucleus, exhibited transcriptional activation activity, and showed
binding to the cis-acting ABA-responsive element (ABRE) in vitro.
In addition, bZIP transcription factors could target on genes by C-Box
and A-box. Except for G-box, the C-box of pathogenic responsive genes
could also bound and negatively regulated by AtbZIP10 inArabidopsis thaliana (Kaminaka et al., 2006). Induced by ABA and
drought, the Tamarix hispida bZIP1 bound to C-box and A-boxcis -elements of the stress response gene (Ji et al., 2013).
In summary, bZIP transcription
factors regulate the transcriptional expression by interacting with
specific cis -regulatory sequences in the promoter region of
response genes to regulate plant stress tolerance (Sornaraj et al.,
2016). To understand the actual relationship between bZIP subfamilies
and their binding cis -regulator motifs (Table I and Figure I),
all the functional annotated bZIPs were categorized into 13 known
subgroups based on the method described by Corrêa et al. (2008). It
seems that the G-Box and ABRE attracts most scientists’ interests and
are two most understood cis -elements of bZIP transcription
factors (Table I). The bZIPs bind to G-Box are categorized into
subfamilies A, C, G, H, K and S; while those recognize ABRE belong to
the subgroups A, B, C, G and S (Table I). Besides, there are also
several reports on mechanisms about how bZIP transcription factors
regulate other two cis -elements, C-box and A-box (Table I).
Interestingly, the bZIPs bind to C-box are usually belong to subfamilies
C and S; the functional annotated bZIP bind to A-box is classified into
subfamily S. Though the number of functional annotated bZIP is limit,
their binding activities of different subfamilies to specificcis -elements could also provide directional suggestions for
further research on de novo bZIPs and potential targets. However, more
evidences are still needed to fulfil the relevance between bZIP
subfamilies and corresponding cis -elements.