The role of phosphorylatable serine residues in the DNA-binding domain of Arabidopsis bZIP transcription factors.

Reversible phosphorylation plays a crucial role in regulating the activity of enzymes and other proteins in all living organisms. Particularly, the phosphorylation of transcription factors can modulate their capability to regulate downstream target genes. In plants, basic domain-containing leucine-zipper (bZIP) transcription factors have an important function in the regulation of many developmental processes and adaptive responses to the environment. By a comprehensive sequence analysis, we identified a set of highly conserved, potentially phospho-accepting serines within the DNA-binding domain of plant bZIPs. Structural modelling revealed that these serines are in physical contact with the DNA and predicts that their phosphorylation will have a major influence on the DNA-binding activity of plant bZIPs. In support of this, we show, by means of a quantitative in vitro binding assay, that phosphorylation-mimicking substitutions of some of these serines strongly interfere with the DNA binding of two prototypical Arabidopsis bZIPs, namely AtZIP63 and HY5. Our data suggest that the identified serines could serve as in vivo targets for kinases and phosphatases, allowing the fine-tuning of bZIP factor activity at the DNA-protein interaction level.