Essential for biological clock functions. Determines the period length of circadian and ultradian rhythms; an increase in PER dosage leads to shortened circadian rhythms and a decrease leads to lengthened circadian rhythms. Essential for the circadian rhythmicity of locomotor activity, eclosion behavior, and for the rhythmic component of the male courtship song that originates in the thoracic nervous system. The biological cycle depends on the rhythmic formation and nuclear localization of the TIM-PER complex. Light induces the degradation of TIM, which promotes elimination of PER. Nuclear activity of the heterodimer coordinatively regulates PER and TIM transcription through a negative feedback loop. Behaves as a negative element in circadian transcriptional loop. Does not appear to bind DNA, suggesting indirect transcriptional inhibition. Required for binding of cwo to the E box regions in the promoters of target genes of the transcriptional activator Clock, probably by binding to Clock-cycle heterodimers, reducing their affinity for E box binding and allowing cwo to bind instead (PubMed:27814361). Forms a heterodimer with timeless (TIM); the complex then translocates into the nucleus. A proportion of the protein exists as homodimer. Nuclear at specific periods of the day. First accumulates in the perinuclear region about one hour before translocation into the nucleus. Interaction with Tim is required for nuclear localization. Additional isoforms seem to exist. Expressed in neural tissues and in several nonneural tissues of the abdomen. Malpighian tubules contain a circadian pacemaker that functions independently of the brain. Expression oscillates in all tissues studied except for the ovary. PER-A isoforms are mainly expressed in adult's head. Expression is sensitive to temperature but not to light. Contains a remarkable run of alternating Gly-Thr residues which is polymorphic in length. At least three types of Gly-Thr length exist in the natural population, (Gly-Thr)23 (shown here), and two major variants (Gly-Thr)17 and (Gly-Thr)20. This Gly-Thr stretch is implicated in the maintenance of circadian period at different temperatures. Deletion of the repeat leads to a shortening of the courtship song cycle period, and thus could be important for determining features of species-specific mating behavior. Mutations in the PAS domain result in longer circadian rhythms and courtship song (PERL mutation) or makes the flies arrhythmic (PER01 mutation). Phosphorylated with a circadian rhythmicity, probably by the double-time protein (dbt). Phosphorylation could be implicated in the stability of per monomer and in the formation of heterodimer per-tim. The tale of a love song - Issue 6 of January 2001