Regulation of Arabidopsis TGA transcription factors by cysteine residues : implication for redox control

Abstract

The Arabidopsis TGA family of basic leucine zipper transcription factors regulate the expression of pathogenesis-related genes and are required for resistance to disease. Members of the family possess diverse properties in respect to their ability to transactivate and interact with NPR1, the central regulator of systemic acquired resistance in Arabidopsis. Two TGA factors, TGA1 and TGA2, have 83 % amino acid similarity but possess differing properties. TGA1 does not interact with NPR1 but is able to transactivate, while TGA2 interacts with NPR1 but is unable to transactivate. This study uses these two TGA factors to identify amino acids that are responsible for their function. Four cysteines residues within TGA1 were targeted for study by site-directed mutagenesis and the resulting mutants were tested for interaction with NPR1 in yeast. The construct containing a mutation of cysteine 260 (Cys-260) interacted well with NPR1, while those with mutations at Cys-172 or Cys-266 interacted poorly. The Cys-260 mutant also displayed the greatest decrease in transactivation potential in yeast, while mutation of Cys-172 or Cys-266 resulted in smaller decreases. Mutation of Cys-287 had no effect on NPR1 interaction or transactivation. Combining various point mutations in a single protein did not increase NPR1 interaction or transactivation levels, indicating that Cys-260 is crucial for regulating TGA1 properties. Cysteines possess the unique ability of forming reversible disulfide bonds which have been shown to regulate several mammalian cellular processes. The observation that mutation of a single TGA1 cysteine (Cys-260) greatly alters the protein’s properties provides a convincing argument that oxidoreduction of this residue is important for its regulation, possibly through the formation of a disulfide bond with either Cys-172 or Cys-266. To test whether other members of the TGA family could be regulated by oxidoreduction, several TGA2 constructs were created that introduced Cys at positions corresponding to those found in TGA1. When tested in yeast none were able to transactivate but continued to interact with NPR1.