Transcriptional corepressor TOPLESS complexes with pseudoresponse regulator proteins and histone deacetylases to regulate circadian transcription.

Circadian clocks are ubiquitous molecular time-keeping mechanisms that coordinate physiology and metabolism and provide an adaptive advantage to higher plants. The central oscillator of the plant clock is composed of interlocked feedback loops that involve multiple repressive factors acting throughout the circadian cycle. Pseudo response regulators (PRRs) comprise a five-member family that is essential to the function of the central oscillator. PRR5, PRR7, and PRR9 can bind the promoters of the core clock genes circadian clock associated 1 (CCA1) and late elongated hypocotyl (LHY) to restrict their expression to near dawn, but the mechanism has been unclear. Here we report that members of the plant Groucho/Tup1 corepressor family, topless/topless-related (TPL/TPR), interact with these three PRR proteins at the CCA1 and LHY promoters to repress transcription and alter circadian period. This activity is diminished in the presence of the inhibitor trichostatin A, indicating the requirement of histone deacetylase for full TPL activity. Additionally, a complex of PRR9, TPL, and histone deacetylase 6, can form in vivo, implicating this tripartite association as a central repressor of circadian gene expression. Our findings show that the TPL/TPR corepressor family are components of the central circadian oscillator mechanism and reinforces the role of this family as central to multiple signaling pathways in higher plants.