Plant bZIP proteins gather at ACGT elements

ACGT cis-acting DNA sequence elements have been identified in a multitude of plant genes regulated by diverse environmental, physiological, and environmental cues. In vivo transient and transgenic plant expression studies have shown that these ACGT elements are necessary for maximal transcriptional activation. Plants possess a conserved family of DNA-binding proteins specific for these DNA sequence motifs. Well-defined in terms of nucleotide sequence required for protein/DNA interactions and transcriptional activation, various ACGT elements have been used as molecular probes to clone sequence-specific DNA binding proteins. All plant DNA-binding proteins specific for ACGT elements belong to the bZIP classification. Most recombinant bZIP proteins can interact with ACGT elements derived from different plant genes, albeit with different affinity. Systematic protein/DNA binding studies have shown that sequences flanking the ACGT core affect bZIP protein binding specificity. These studies have provided the basis for a concise ACGT nomenclature and defined high-affinity A-box, C-box, and G-box elements. Plant bZIP factors can be classified according to their quantitative binding affinities for high-affinity ACGT elements. Potential molecular mechanisms that may control how plant bZIP proteins activate plant gene expression are discussed.