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 pl ant defense responses. To gain a better understanding of NPR1 function, we conducted a yeast two-hybrid screening procedure and identified a different ial interaction between NPR1 and all known members of the Arabidopsis TGA f amily 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 th e pathogenesis-related PR-1 gene. Proteins encoded by npr1 mutants interact ed 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 di sease resistance and the loss of TGA2 interaction and NPR1-enhanced DNA bin ding. Coupled with observations that the DNA binding activity of TGA factor s is deregulated in npr1 plants, the results suggest that NPR1-mediated DNA binding of TGA2 is critical for activation of defense genes.