The Drosophila PAR domain protein 1 (Pdp1) gene encodes a transcription factor with multiple functions. One isoform, PDP1ε, was proposed to be an essential activator of the core clock gene, Clock (Clk). However, a central clock function for PDP1ε was recently disputed, and genetic analysis has been difficult due to developmental lethality of Pdp1-null mutants. Here we report the discovery of a mutation that specifically disrupts the Pdp1εisoform. Homozygous Pdp1εmutants are viable and exhibit arrhythmic circadian behavior in constant darkness and also in the presence of light:dark cycles. Importantly, the mutants show diminished expression of CLK and PERIOD (PER) in the central clock cells. In addition, expression of PDF (pigment-dispersing factor) is reduced in a subset of the central clock cells. Loss of Pdp1εalso alters the phosphorylation status of the CLK protein and disrupts cyclic expression of a per-luciferase reporter in peripheral clocks under free-running conditions. Transgenic expression of PDP1ε in clock neurons of Pdp1εmutants can restore rhythmic circadian behavior. However, transgenic expression ofCLKin these mutants rescues the expression ofPERin the central clock, but fails to restore behavioral rhythms, suggesting that PDP1ε has effects outside the core molecular clock. Together, these data support a model in which PDP1ε functions in the central circadian oscillator as well as in the output pathway.
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