1. Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208, USA
2. Department of Pathology, Northwestern University, Evanston, Illinois 60208, USA
3. Howard Hughes Medical Institute, Brandeis University, Waltham, Massachusetts 02454, USA
4. These authors contributed equally to this work

Correspondence to: Ravi Allada^1,4 Correspondence and requests for materials should be addressed to R.A. (e-mail: Email: r-allada@northwestern.edu).

Abstract

Circadian clocks drive rhythmic behaviour in animals and are regulated by transcriptional feedback loops^{1, 2}. For example, the Drosophila proteins Clock (Clk) and Cycle (Cyc) activate transcription of period (per) and timeless (tim). Per and Tim then associate, translocate to the nucleus, and repress the activity of Clk and Cyc. However, post-translational modifications are also critical to proper timing. Per and Tim undergo rhythmic changes in phosphorylation¹, and evidence supports roles for two kinases in this process: Doubletime (Dbt) phosphorylates Per^{3, 4}, whereas Shaggy (Sgg) phosphorylates Tim⁵. Yet Sgg and Dbt often require a phosphoserine in their target site^{6, 7}, and analysis of Per phosphorylation in dbt mutants^{3, 8} suggests a role for other kinases. Here we show that the catalytic subunit of Drosophila casein kinase 2 (CK2) is expressed predominantly in the cytoplasm of key circadian pacemaker neurons. CK2 mutant flies show lengthened circadian period, decreased CK2 activity, and delayed nuclear entry of Per. These effects are probably direct, as CK2 specifically phosphorylates Per in vitro. We propose that CK2 is an evolutionary link between the divergent circadian systems of animals, plants and fungi.