Scavenging for the transcription activation sequence in the SOG1 disordered regions

Eukaryotic organisms rely on the evolutionary conserved DNA damage response mechanism to maintain genome integrity. Despite sharing many DNA damage response components, plants lack the key mammalian regulator called tumor suppressor P53. Instead, they use the functionally equivalent but otherwise unrelated plant-specific transcription factor SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1). Outside the conserved folded DNA-binding NAC domain, SOG1 is disordered and carries a net negative charge in its C-terminal tail. Using a high-throughput screening method in yeast, we localized the SOG1 transcription activation domain (TAD) to an acidic C-terminal helical region that is flanked by regulatory phosphorylation sites. Mutational analysis suggests phosphorylation increases TAD activity. Not only does this finding categorize SOG1 as compliant with the Acidic Exposure Model, it also extends the model: negative charges originating from both acidic residues (Asp and Glu) and phosphorylation sites expose key hydrophobic residues to the solvent where they interact with binding partners. Furthermore, the TAD of SOG1 relatives in and outside the Arabidopsis genus were identified. Finally, expression and purification of the recombinant SOG1 C-terminal helical region encompassing the TAD allowed for biophysical characterization of the TAD.