Rhythmic and light-inducible appearance of clock-associated pseudo-response regulator protein PRR9 through programmed degradation in the dark in Arabidopsis thaliana.

In Arabidopsis thaliana, it is currently believed that the members of a small family of PSEUDO-RESPONSE REGULATOR (PRR) proteins, including TOC1 (TIMING OF CAB EXPRESSION 1), coordinately play roles close to the circadian clock. Among these PRR members, the PRR9 gene is unique in that not only does its transcription oscillate diurnally, but it is also rapidly induced by light in a manner dependent on phytochromes. These events at the level of transcription must be crucial for the clock-associated functions of PRR9. Nonetheless, little is known about the expression of the PRR9 protein product itself in plant cells. Here, we show that PRR9 polypeptides themselves oscillate diurnally, and that they accumulate rapidly in response to light. Our work further suggests that the presence of PRR9 polypeptides is controlled through proteasome-mediated programmed degradation in the dark.