Gene/Protein Structure of C-Repeat binding Transcription Factor

Table of content 

• Gene/Protein Structure of C- Repeat Binding Transcription Factor
• CBF Gene family
• Function of DREBs/CBF (C-repeat binding factor)

Gene/Protein Structure of C- Repeat Binding Transcription Factor

CBF Gene family

CBF (C-repeat bindingfactor) also known  as DREB (dehydration-responsive element-binding protein). DREB is the class of transcription factors in plants that induces under stress stimuli, and involve in the regulation of series of cascade for the activation of stress responsive gene to bring tolerance in plants.

The CBF (CBF1, CBF2, and CBF3) genes in Arabidopsis organize in tandem on chromosome 4. Both CBF1 and CBF2 induced by cold, while CBF4 involved in drought resistance. It is difficult to determine the chromosomal location for a gene in plant of complex genome and where there high variation in chromosomal ploidy and aneuploidy level like sugarcane. However in a recent study about 110 SsDREB subfamily proteins identified in S. spontaneum. Which were classified into six groups, the number of identified protein is also greater than other species. The SsDREB genes variably distributed across all 29 chromosomes of S. spontaneum, while chromosome 2B contain the greater number of DREB genes.

The first cDNA of CBF1 (C-repeat/DREB Factor 1) was isolated from Arabidopsis thaliana in 1997 by Stockinger and colleague. Molecular mass of the protein is about 24 KDa, comprises of nuclear localization domain and acidic activation domain.  All AP2/ERF family members have conserved AP2 DNA-binding motif that is compose of ~ 60 amino acids residue.  AP2 DNA-binding domain recognize and bind to specific sequence of cis-regulatory elements present in promoters of many stress responsive gene  The AP2 domain further possess two regions: 1)-YRG region/element that consists 20 amino acids, it is present in N-terminal stretch and is rich with hydrophobic and basic amino acids, responsible for DNA-binding activities. 2)-The RAYD region/element contains 40 amino acids. At RAYD region about 18-amino-acid stretch is present at C-terminal and is responsible for amphipathic alpha helix formation, that involve in protein–protein interaction. Three-dimensional analysis by using nuclear magnetic resonance (NMR) of AP2/ERF motif/domain revealed a β-sheets having three strands that connected anti-parallel way by α-helix and loops. Similarly, at AP2/ERF domain's end the DSAW motif is present and at the C-terminal region end LWSY motif is present. However nuclear localization signal (NLS) involved in entrance of DREB proteins in nucleus, it is a stretch rich of basic-amino acid residues. Those TF proteins without the presence of NLSs enter the nucleus by protein–protein interaction with the help of TFs that have NLSs. In AP2/ERF protein domain at position 19 the glutamic acid and at position 14 amino acid Valine are quite conserved but, some species amino acid at 19 is replace by valine .

Representation of typical structure of CBF/DREB protein domain.

Function of DREBs/CBF (C-repeat binding factor)

The DREBs/CBF (C-repeat binding factor), belongs to ERF subfamily and play a key role in plants in response to stress stimuli. The DREB/CBF gene are mainly involved in expression of cold stress-responsive genes. Along with cold stress DREB greatly express under other abiotic stress like drought, salinity in ABA dependently and ABA independently. CBF family further includes three members namely DREB1a/CBF3, DREB1b/CBF1, and DREB1c/CBF2 that induce gene transcription of Cold-regulated genes (COR) under cold stress. The CBF TFs recognized and bind to the conserved CRT/DRE motif CCGAC present in the promoters of many cold-regulated (COR) genes which mediate plant freezing tolerance. In sugarcane CBF/DREB greatly expresses under drought and cold stresses and mediate wide range of COR gene that confer tolerance in many plant. Currently several inducer of CBF have been identified which includes MYB, Inducer of CBF expression (ICE1 and ICE2), Ethylene insensitive (EIN3), Calmodulin-binding transcription activator (CAMTA1, CAMTA2, CAMTA3), Phyto-chrome-interacting (PIF3/4/7) factors, Suppressor of overexpression of co1 (SOC1) and others. Which regulate the expression of CBF and continue the cold signaling pathway. Nevertheless ICE1-CBF-COR best known signaling cascade involve plants in cold tolerance responses.

Low temperature CBFs activation, CBFs bind to CRT/DRE cis-acting element, present in promoter of COR gene, activation of whole signaling cascade results in an increase freezing tolerance.