The Arabidopsis NPR1/NIM1 Protein Enhances the DNA Binding Activity of a Subgroup of the TGA Family of bZIP Transcription Factors

The Arabidopsis NPR1 gene is essential in activating systemic, inducible plant defense responses. To gain a better understanding of NPR1 function, we conducted a yeast two-hybrid screening procedure and identified a differential interaction between NPR1 and all known members of the Arabidopsis TGA family of basic leucine zipper transcription factors. In the electrophoretic mobility shift assay, NPR1 substantially increased the binding of TGA2 to its cognate promoter element (as-1) as well as to a positive salicylic acid-inducible element (LS7) and a negative element (LS5) in the promoter of the pathogenesis-related PR-1 gene. Proteins encoded by npr1 mutants interacted poorly with TGA2 and did not substantially increase TGA2 binding to the as-1, LS5, or LS7 elements, thus establishing a link between the loss of disease resistance and the loss of TGA2 interaction and NPR1-enhanced DNA binding. Coupled with observations that the DNA binding activity of TGA factors is deregulated in npr1 plants, the results suggest that NPR1-mediated DNA binding of TGA2 is critical for activation of defense genes.