CONSTRUCTION OF A TRANSDOMINANT INHIBITOR FOR MEMBERS OF THE TGA FAMILY OF TRANSCRIPTION FACTORS CONSERVED IN HIGHER-PLANTS

Activating sequence factor 1 (ASF-1) is a conserved DNA-binding activi ty that interacts with sequence elements containing TGACG motifs, some of which have been demonstrated to respond to exogenous application o f auxin and salicylic acid. Genes encoding transcription factors with similar DNA-binding specificity to ASF-1 have been cloned from diverse plant species and these factors all contain a distinct basic-leucine- zipper (bZIP) motif. Members of this family of DNA-binding proteins, d esignated as TGA factors, have been shown to interact with similar DNA sequences and at least seven distinct TGA genes are present in the Ar abidopsis genome. To study the roles that this family of factors may p lay in plant development, a trans-dominant inhibitor of ASF-1 was cons tructed by deleting the basic portion of the bZIP domain in the tobacc o factor TGA1a. In vitro co-expression studies demonstrated that this deletion mutant, named TGA1a-D, suppresses the DNA-binding activity of wildtype tobacco TGA1a, and three different members of the Arabidopsi s TGA family. In contrast, co-expression of TGA1a-D with another class of bZIP proteins, the G-Box Binding Factor family, showed no suppress ion of DNA-binding activity. Over-expression of TGA1a-D in transgenic tobacco significantly decreased nuclear ASF-1 relative to several othe r known factors, indicating that the proteins comprising ASF-1 activit y in vivo are likely TGA family members. Thus, TGA1a-D may be a family -specific inhibitor for the TGA family and should facilitate the study of ASF-1 function in vivo.