PDP1 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 (Pdp1 ) transcription, then sequential repression by VRI and activation by PDP1 mediate rhythms in Clk transcription. Because VRI and PDP1 bind the same regulatory element, the VRI/PDP1 ratio is thought to control the level of Clk transcription. Thus, constant high or low PDP1 levels in clock cells should eliminate Clk mRNA cycling and disrupt circadian oscillator function. Here we show that reducing PDP1 levels in clock cells by 70% via RNA interference or increasing PDP1 levels by 10-fold in clock cells does not alter Clk mRNA cycling or circadian oscillator function. However, constant low or high PDP1 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 ratio neither controls Clk mRNA cycling nor circadian oscillator function and argue that PDP1 is not essential for Clk activation. PDP1 is nevertheless required for behavioral rhythmicity, which suggests that it functions to regulate oscillator output.