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 (By similarity). Forms a heterodimer with timeless (TIM); the complex then translocates into the nucleus. 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 (By similarity). The run of Gly-Thr 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 (By similarity). 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 (By similarity). The sequence shown here is that of clones MA-1, MA-2, MA-4 and MA-5.