Developmental and tissue-specific regulation of the gene for the wheat basic/leucine zipper protein HBP-1a(17) in transgenicArabidopsis plants

Wheat basic/leucine zipper protein HBP-1a(17) binds in vitro specifically to ACGT motif-containing cis-acting elements, such as the type I element of plant histone promoters and the G-box of hormone- and light-inducible promoters. To address the in vivo function of HBP-1a(17), we isolated and structurally analyzed theHBP-1a(17) gene and examined its expression in transgenicArabidopsis plants. TheHBP-1a(17) gene is composed of 14 exons; the basic region and leucine zipper are encoded by separate small exons, as is the case for other bZIP protein genes. The G-box of theHBP-1a(17) promoter bound specifically to HBP-1a(17) and its related HBP-1a isoforms, suggesting that theHBP-1a(17) gene may be autoregulated, although the binding affinity of these proteins in vitro is very low. InArabidopsis plants, activation of theHBP-1a(17) promoter was highly restricted to photosynthetically active mesophyll, and guard cells and vascular bundles of vegetative leaves. Etiolation of transgenic plants resulted in inhibition of expression of theHBP-1a(17) promoter. Indeed, theHBP-1a(17) promoter contains several sequence elements homologous to cis-acting elements conserved in light-inducible promoters. It is, therefore, assumed that theHBP-1a(17) gene is light regulated and that HBP-1a(17) is involved in light-responsive gene transcription via the G-box. Communicated by K. Isono