Isolation and molecular characterization of PosF21, an Arabidopsis thaliana gene which shows characteristics of a b‐Zip class transcription factor.

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Abstract
We have isolated a putative transcription factor gene, PosF21, from Arabidopsis thaliana using an indirect cross‐hybridization approach. cDNA clones were isolated which encode single repeating amino acids. Such sequences may function as activation domains in transcription factors and may be indicative for such proteins. The clone PosF21 encodes a region very rich in glutamines. Besides this putative activation domain it encodes a protein sequence which shows all the characteristics of a basic‐domain/leucine zipper type of DNA‐binding domain. PosF21 is expressed constitutively at a low level in young seedlings and in roots, stems and leaves of mature Arabidopsis plants. A genomic clone of PosF21 was isolated and the gene structure was analyzed. Related sequences in Arabidopsis and a wide range of other plants were detected using the putative DNA‐binding domain as a probe in cross‐hybridization experiments. Transient transformations in tobacco protoplasts were performed using the beta‐glucuronidase (GUS) gene as reporter gene. Approximately 400 bp of the 5' genomic region of PosF21 promote expression of the GUS gene in tobacco protoplasts. Evidence for a regulatory function of PosF21 was obtained since co‐ expression of the full PosF21 protein or its DNA‐binding region alone specifically stimulated GUS gene expression directed from the PosF21 promoter by 6‐8‐fold.