The Drosophila CLOCK protein is constantly present in both timekeeping and other cells

CLK is coexpressed with PERIOD in oscillator cells throughout the head and body of Drosophila.

• CLK binds E-boxes to activate the transcription of genes associated with the circadian clock, even at low levels.
• Recent findings suggest that CLK may also influence nonrhythmic gene expression and behaviors.
• CLK is primarily localized in the nucleus and maintains consistent levels throughout the circadian cycle.
• The expression of CLK is observed in non-oscillator cells in the brain, including Kenyon cells linked to learning and memory.
• These observations indicate that CLK's stable presence in the nucleus could regulate rhythmic transcription and other behavioral processes.

Simplified