Tobacco TGA factors differ with respect to interaction with NPR1, activation potential and DNA-binding properties

In higher plants, as-1-like cis elements mediate auxin- and salicylic acid- inducible transcription. Originally found in viral and T-DNA promoters, the y are also functional elements of plant promoters activated during the defe nce response against pathogens. Tobacco bZIP transcription factor TGA1a was the first recombinant protein shown to bind to as-1. cDNAs for two novel t obacco as-1-binding bZIP proteins (TGA2.1 and TGA2.2) were isolated. Reveal ing a high degree of amino acid identity in the bZIP domain (89%) and the C -terminus (79%), the two TGA2 factors differ remarkably with respect to the length of the N-terminus (170 amino acids in TGA2.1 versus 43 amino acids in TGA2.2). TGA2.1 and TGA2.2, but not TGA1a, interacted with ankyrin repea t protein NPR1, a central activator of the plant defence response. In contr ast, TGA2.1 and TGA1a, but not TGA2.2, functioned as transcriptional activa tors in yeast. Apart from conferring transcriptional activation, the N-term inal domain of TGA2.1 led to reduced in vitro as-1-binding activity and alm ost completely abolished binding to one half site of this bifunctional elem ent. When being part of a heterodimer with TGA2.2, TGA2.1 was efficiently r ecruited to a single half site, though double occupancy of the element was still preferred. In contrast, TGA1a preferred to bind to only one palindrom e, a feature that was also maintained in heterodimers between TGA1a and TGA 2.1 or TGA2.2.