Abstract
Circadian (about one-day) rhythm is endogeneously generated and self-sustained system that makes organisms to adjust to daily changes in environment. Genetic approaches have identified clock-associated genes in Arabidopsis. However, little knowledge about biochemical properties of clock-associated proteins has prevented us to understand the clock system in molecular level.
In this talk, we will present the molecular function of PSEUDO-RESPONSE REGULATOR 9, 7, and 5 proteins as transcriptional repressors in the clock. Using a glucocorticoid-induced PRR5-GR construct, we found that PRR5 directly down-regulates CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) expression. PRR9, PRR7, and PRR5 suppressed CCA1 and LHY promoter activities, and gave transcriptional repressor activity to a heterologous DNA binding protein. Furthermore, PRR9, PRR7, and PRR5 associated with the CCA1 and LHY promoters in vivo from early daytime to midnight, coinciding with the time these genes were repressed. In turn, CCA1 and LHY activate PRR9 and PRR7 expression in the morning. We propose that the negative feedback system with time lag of several hours constitutes circadian rhythm.
