Abstract
An interlocking transcriptional-translational 24-h-feedback loop of clock-associated genes is thought to be the clock central oscillator in plants. Genetic studies have suggested that PSEUDO-RESPONSE REGULATOR 9 (PRR9), PRR7, and PRR5 act within or close to the loop, however their molecular functions remain unknown. Here we demonstrate that PRR9, PRR7, and PRR5 act as transcriptional repressors of CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY). Each PRR9, PRR7, and PRR5 suppresses CCA1 and LHY promoter activities, and can give transcriptional repressor activity to a heterologous DNA binding protein in a transient reporter assay. Using a glucocorticoid-induced PRR5-GR construct, we found that PRR5 directly down-regulates CCA1 and LHY expression. Furthermore, PRR9, PRR7, and PRR5 associate with the CCA1 and LHY promoters in vivo from early daytime to midnight, coinciding with the time these genes are repressed. These results suggest that the repressor activities of PRR9, PRR7 and PRR5 on the CCA1 and LHY promoter regions constitute the molecular mechanism that accounts for the role of these proteins in the feedback loop of the circadian clock.
