Molecular dissection of the promoter of the light-induced and circadian-controlled APRR9 gene encoding a clock-associated component of Arabidopsis thaliana.

In the model higher plant Arabidopsis thaliana, a number of circadian clock-associated protein components have recently been identified. Among them, a small family of ARABIDOPSIS PSEUDO-RESPONSE REGULATORS (APRR1/TOC1, APRR3, APRR5, APRR7, and APRR9) is interesting because the most probable clock component TIMING OF CAB EXPRESSION 1 (TOC1) belongs to this family. Several lines of evidence have already been accumulated to support the view that not only APRR1/TOC1 but also other APRR family members are crucial for a better understanding of the molecular link between circadian rhythm and light-signal transduction. Among the APRR1/TOC1 family members, the circadian-controlled APRR9 gene is unique in that its expression is rapidly induced by light at the level of transcription. In this study we dissected the regulatory cis-elements of the light-induced and/or circadian-controlled APRR9 promoter by employing not only a mutant plant carrying a T-DNA insertion in the APRR9 promoter, but also a series of APRR9-promoter::LUC (luciferase) reporters that were introduced into an Arabidopsis cultured cell line (T87 cells). Taking the results of these approaches together, we provide several lines of evidence that the APRR9 promoter contains at least two distinctive and separable regulatory cis-elements: an "L element" responsible for the light-induced expression, followed by an "R element" necessary for the fundamental rhythmic expression of APRR9. Furthermore, APRR1/TOC1 was implicated in the L-element-mediated light response of APRR9, directly or indirectly.