THE TOBACCO TRANSCRIPTION ACTIVATOR TGA1A BINDS TO A SEQUENCE IN THE 5' UPSTREAM REGION OF A GENE ENCODING A TGA1A-RELATED PROTEIN

We have isolated and characterized a tobacco gene, designated G13, encoding a leucine zipper DNA-binding protein related to the transcription activator TGA1a. The G13 coding region is divided into eight exons and the amino acid sequence of the encoded protein (PG13) shows 76% homology to TGA1a. Their putative DNA-contacting regions (basic domains) are identical and they both bind to the same target sequences in vitro. By contrast, some differences are apparent between these proteins at the carboxyl end of the dimerization region (leucine zipper). The basic and leucine zipper domains are encoded on separate small exons. Analysis by DNAse I footprinting, gel shift and competition experiments revealed that TGA1a and PG13 synthesized in Escherichia coli, and the tobacco nuclear factor ASF-1 all bind to at least one site in the 5' upstream region of G13. The presence of a TGA1a binding site in the upstream region of a TGA1a-related gene suggests that transcription of this gene is autoregulated.