TGA3 IS A DISTINCT MEMBER OF THE TGA FAMILY OF BZIP TRANSCRIPTION FACTORS IN ARABIDOPSIS-THALIANA

TGA1a is a well-characterized transcription factor that may mediate th e root-specific and auxin-responsive expression of some plant genes. I n tobacco, Southern blot and genomic cloning analyses have shown that TGA1a consists of at least four closely related genes. Since TGA1a bel ongs to the bZIP class of transcription factors, the protein products of the tobacco TGA1a family are likely to form hetero-dimers with each other in addition to the homo-dimers. In order to find a model plant system that may have less genomic complexity, we have now characterize d a TGA1a-related gene (TGA3) from Arabidopsis thaliana. Southern blot analyses at high stringency suggest that Arabidopsis contains only on e copy of TGA3 per haploid genome. However, low stringency Southern bl ot analyses with homologous and heterologous probes suggest that there is a multigene family of TGA1a-related genes present in Arabidopsis, of which TGA1, TGA2 and TGA3 are members. Although these gene members share a highly conserved bZIP region, they are not genes with high hom ologies at the nucleotide level. Similar to TGA1a of tobacco, TGA3 is most highly expressed in root tissues and recombinant TGA3 protein sho ws similar DNA-binding site specificity to that of TGA1a in vitro. Com parison of the genomic organization between TGA3 and the tobacco homol ogue PG13 reveals striking conservation in the sizes and positions of exons and introns in the region surrounding the bZIP domain.