Endogenous circadian pacemaker anticipates the daily periodic environmental changes,orchestrates many biochemical and physiological processes accordingly and provides an adaptive advantage.Pseudo-response regulators(PRRs)are key components of transcription-translation feedback loops of circadian networks in green alga and plants.The Arabidopsis PRRs gene family consists of five members(PRR9,PRR7,PRR5,PRR3,and TOC1/PRR1)which are expressed with peaks staggered from early morning to dusk.The most interesting and confusing aspects of PRRs single and double mutants are their variety of circadian phenotypes.We determined that all PRR proteins work as central repressors of CCA1 and have similar function to circadian clock by switching either the promoters or CDS regions of PRRs.We further demonstrate that the clockregulated phase-specific expression of PRRs is essential to their circadian function.We also show that the roughly 12-hour translational dynamics of PRRs is the key to understand why plant circadian displays an endogenous oscillation of about 24 hours,and then give a reasonable hypothesis to explain the short or long period phenotypes of PRRs mutants.