ARCHITECTURE CHARACTERISTICS OF BZIP TRANSCRIPTION FACTORS
Transcription factor (TF), also known as trans -acting factor, is a category of proteins that can specifically bind to cis -acting elements in the promoter region of eukaryotic genes, thereby activating or silencing the expression of related genes in a specific time and space. The structure of plant transcription factors generally includes at least four functional domains, including the DNA binding domain, the transcriptional regulatory domain, the nuclear localization signal and the oligomerization site (Du et al., 2012). They work together to regulate various biological processes.
Although the classification of bZIP varies depending on the researcher’s choice, there is currently a consensus on this type of transcription factor that the protein sequence contains a conserved bZIP domain with 60-80 amino acids length. This domain is consisted of at least two specific structures. Firstly, the N-terminus is a basic region composed of about 20 basic amino acids, containing a nuclear localization signal (NLS) and a N-x7-R/K structural unit that specifically binds to a DNA sequence. This region is involved in nuclear localization and DNA binding (Lee et al., 2006b). Secondly, the C-terminus, which is a leucine zipper region, a heptad repeat of leucine or other bulky hydrophobic amino acids (Ile, Val, Phe, or Met) positioned exactly nine amino acids towards the C-terminus, creating an amphipathic helix. This region is involved in the dimerization of the bZIP protein before it binds to DNA (Hurst, 1994; Jakoby et al., 2002; Landschulz et al., 1988). In addition to the bZIP conserved domain, the bZIPs also contain other domains with transcriptional activation functions, such as the R/KxxS/T and S/TxxD domains, which are phosphorylation sites of Ca2+ independent protein kinase and casein kinase II (Furihata et al., 2006). Besides, there are also some regions rich in acidic amino acids such as Pro and Gln, which can activate the transcriptional expression of downstream target genes (Liao et al., 2008).