Dynamic PER repression mechanisms in the Drosophila circadian clock: from on-DNA to off-DNA

Abstract

Transcriptional feedback loops are central to the generation and maintenance of circadian rhythms. In animal systems as well as Neurospora, transcriptional repression is believed to occur by catalytic post-translational events. We report here in the Drosophila model two different mechanisms by which the circadian repressor PERIOD (PER) inhibits CLOCK/CYCLE (CLK/CYC)-mediated transcription. First, PER is recruited to circadian promoters, which leads to the nighttime decrease of CLK/CYC activity. This decrease is proportional to PER levels on DNA, and PER recruitment probably occurs via CLK. Then CLK is released from DNA and sequestered in a strong, ∼1:1 PER–CLK off-DNA complex. The data indicate that the PER levels bound to CLK change dynamically and are important for repression, first on-DNA and then off-DNA. They also suggest that these mechanisms occur upstream of post-translational events, and that elements of this two-step mechanism likely apply to mammals.