DNA-BINDING PROPERTIES, GENOMIC ORGANIZATION AND EXPRESSION PATTERN OF TGA6, A NEW MEMBER OF THE TGA FAMILY OF BZIP TRANSCRIPTION FACTORS IN ARABIDOPSIS-THALIANA

The TGA genes encode a family of basic domain-leucine zipper (bZIP) tr anscription factors that are conserved in higher plants. We have conti nued to unravel the complexity of this gene family by using a polymera se chain reaction (PCR)-based approach. Taking advantage of the conser ved amino acid sequence in the bZIP domain found in all members of thi s gene family, two degenerate oligonucleotides were synthesized based on the sequence of this region in order to amplify by PCR the analogou s genomic fragments from the various TGA loci in Arabidopsis. This app roach has led us to the finding of a new member of the TGA gene family , and subsequently the isolation of a gene designated as TGA6. Further characterization of the TGA6 locus confirmed our prediction that the gene structure of this family is remarkably conserved. Genomic Souther n blot analysis revealed that TGA6 is a single-copy gene in Arabidopsi s. Based on the genomic sequence information, gene-specific primers we re synthesized for isolating the cDNA that corresponds to the coding r egion. Subsequently, the cDNA for TGA6 was cloned and sequenced. Gel m obility shift assays with in vitro translated TGA6 protein showed that TGA6 is more like TGA5 in terms of its in vitro DNA-binding propertie s. The expression of TGA6 in different tissues was estimated by using reverse transcriptase (RT)-PCR and further analyzed in transgenic Arab idopsis lines expressing a TGA6 promoter-GUS fusion. TGA6 promoter act ivity is found primarily in roots of young seedlings. As seedlings dev elop, TGA6 is expressed in aging cotyledons, mesophyll cells of hydath odes on leaf margins, vascular tissue and trichomes of senescing roset te leaves, but is very low in primary roots of mature plants. High lev els of expression persist in young lateral roots and in regions of the primary root where lateral roots are emerging. In flowers, the activi ty is localized predominantly to mature pollen grains. The expression pattern of TGA6 reported here is strikingly similar to that of an Arab idopsis acidic chitinase gene. Possible biological significance of TGA 6 in cellular defense against pathogens and abiotic stress is discusse d.