Combinatorial interaction of cis elements specifies the expression of the Arabidopsis AtHsp90-1 gene.

The promoter region of the Arabidopsis AtHsp90-1 gene is congested with heat shock elements and stress response elements, as well as with other potential transcriptional binding sites (activating protein 1, CCAAT/enhancer-binding protein element, and metal regulatory element). To determine how the expression of this bona fide AtHsp90-1 gene is regulated, a comprehensive quantitative and qualitative promoter deletion analysis was conducted under various environmental conditions and during development. The promoter induces gene expression at high levels after heat shock and arsenite treatment. However, our results show that the two stress responses may involve common but not necessarily the same regulatory elements. Whereas for heat induction, heat shock elements and stress response elements act cooperatively to promote high levels of gene expression, arsenite induction seems to require the involvement of activating protein 1 regulatory sequences. In stressed transgenic plants harboring the full-length promoter, beta-glucuronidase activity was prominent in all tissues. Nevertheless, progressive deletion of the promoter decreases the level of expression under heat shock and restricts it predominantly in the two meristems of the plant. In contrast, under arsenite induction, proximal sequences induce AtHsp90-1 gene expression only in the shoot meristem. Distally located elements negatively regulate AtHsp90-1 gene expression under unstressed conditions, whereas flower-specific regulated expression in mature pollen grains suggests the prominent role of the AtHsp90-1 in pollen development. The results show that the regulation of developmental expression, suppression, or stress induction is mainly due to combinatorial contribution of the cis elements in the promoter region of the AtHsp90-1 gene.