When phosphorylated by an unknown kinase, PRR5 and PRR3 proteins demonstrate increased binding to TIMING OF CAB2 EXPRESSION 1 ( TOC1). This interaction stabilizes both TOC1 and PRR5 and prevents their degradation by the F-box protein ZEITLUPE (ZTL). Through this mechanism, PRR5 is indirectly activated by light, as ZTL is inhibited by light. Additionally, PRR5 contributes to the transcriptional repression of the genes encoding the single MYB transcription factors CCA1 and LHY.