Transcription factor that is essential for the generation of the circadian clock oscillation. Is necessary for activation of CCA1 and LHY expression. Is coregulated with TOC1 and seems to be repressed by CCA1 and LHY by direct binding of these proteins to the evening element in the LUX promoter. Directly regulates the expression of PRR9, a major component of the morning transcriptional feedback circuit, by binding specific sites on PRR9 promoter. Binds to its own promoter, inducing a negative auto-regulatory feedback loop within the core clock. Binds to ELF3 and associates with ELF4 in a diurnal complex which is required for the expression of the growth-promoting transcription factors PIF4 and PIF5 and subsequent hypocotyl growth in the early evening. Interacts with ELF3 and forms a complex with ELF3 and ELF4. A number of isoforms are produced. According to EST sequences. Circadian oscillation with peaks at subjective dusk. Mutant plant flowering lacks a photoperiodic response.