Cis-combination of the classic perS and perL mutations results in arrhythmic Drosophila with ectopic accumulation of hyperphosphorylated PERIOD protein

The 1st circadian "clock" gene identified was the X-linked period (per) gene in Drosophila melanogaster. In the pioneering initial report, Konopka and Benzer (1971) characterized 3 alleles of per that shortened (per^S; ∼19 h), lengthened (per ^L; ∼29 h), or abolished (per⁰) circadian behavioral rhythms. They also showed that transheterozygotes carrying the per^S and per^L mutations exhibit robust behavioral rhythms with nearly normal periods of ∼23 h, highlighting the semidominant nature of many clock mutants. In this study, per⁰ flies bearing a doubly mutated per transgene that carries both the per ^S and per^L alleles (per⁰; per ^S/L) were analyzed for behavioral and molecular rhythms. Unlike singly mutated versions, the per⁰;per^S/L transgenic flies are arrhythmic in constant dark conditions and exhibit little, if any, entrainment to daily light-dark cycles. In a wildtype per⁺ background, expression of per^S/L abolishes behavioral rhythms, indicating that it functions in a transdominant negative fashion. Biochemical analysis of head extracts revealed that only hyperphosphorylated isoforms of the PERS/L protein are detected throughout a daily cycle, and the levels remain constant. Intriguingly, little if any PERS/L is observed in key pacemaker neurons that control daily activity rhythms, consistent with the notion that hyperphosphorylated isoforms of PER are unstable. Nonetheless, PERS/L is detected in ectopic cells in the brain, in which it exhibits an unusual localization, mainly staining the periphery of the nucleus. These results suggest that posttranslational mechanisms play a key role in limiting the accumulation of PER to specific cells. On a broader scope, our results indicate that the semidominant effects of period-altering alleles observed in trans are not necessarily preserved in the cis-configuration and that novel phenotypes can emerge.