The timSL mutant affects a restricted portion of the Drosophila melanogaster circadian cycle.

The circadian rhythm genes period (per) and timeless (tim) are central to contemporary studies on Drosophila circadian rhythms. Mutations in these genes give rise to arrhythmic or period-altered phenotypes, and per and tim gene expression is under clock control. per and tim proteins (PER and TIM) also undergo circadian changes in level and phosphorylation state. The authors previously described a period-altering tim mutation, timSL, with allele-specific effects in different per backgrounds. This mutation also affected the TIM phosphorylation profile during the mid-late night. The authors show here that the single amino acid alteration in TIM-SL is indeed responsible for the phenotype, as a timSL transgene recapitulates the original mutant phenotype and shortens the period of perL flies by 3 h. The authors also show that this mutation has comparable effects in a light-dark cycle, as timSL also accelerates the activity offset during the mid-late night of perL flies. Importantly, timSL advances predominantly the mid-late night region of the perL phase response curve, consistent with the notion that this portion of the cycle is governed by unique rate-limiting steps. The authors propose that TIM and PER phosphorylation are normally rate determining during the mid-late night region of the circadian cycle.