PDP1ε functions downstream of the circadian oscillator to mediate behavioral rhythms

The Drosophila circadian oscillator is composed of autoregulatory period/timeless (per/tim) and Clock (Clk) feedback loops that control rhythmic transcription. In the Clk loop, CLOCK-CYCLE heterodimers activate vrille (vri) and PAR domain protein 1 epsilon (Pdp1 epsilon) transcription, then sequential repression by VRI and activation by PDP1 epsilon mediate rhythms in Clk transcription. Because VRI and PDP1 epsilon bind the same regulatory element, the VRI/PDP1 epsilon ratio is thought to control the level of Clk transcription. Thus, constant high or low PDP1 epsilon levels in clock cells should eliminate Clk mRNA cycling and disrupt circadian oscillator function. Here we show that reducing PDP1 epsilon levels in clock cells by similar to 70% via RNA interference or increasing PDP1 epsilon levels by similar to 10-fold in clock cells does not alter Clk mRNA cycling or circadian oscillator function. However, constant low or high PDP1 epsilon levels in clock cells disrupt locomotor activity rhythms despite persistent circadian oscillator function in brain pacemaker neurons that extend morphologically normal projections into the dorsal brain. These results demonstrate that the VRI/PDP1 epsilon ratio neither controls Clk mRNA cycling nor circadian oscillator function and argue that PDP1 epsilon is not essential for Clk activation. PDP1 epsilon is nevertheless required for behavioral rhythmicity, which suggests that it functions to regulate oscillator output.