CikA, a Bacteriophytochrome That Resets the Cyanobacterial Circadian Clock

TLDR

The circadian oscillator of *Synechococcus elongatus* is entrained by environmental cues, similar to eukaryotic clocks. The study investigates CikA as a key component of the environmental input pathway to the cyanobacterial circadian oscillator. Inactivation of cikA shortens the circadian period by ~2 hours, alters rhythm phasing, and nearly abolishes darkness‑induced phase resetting, while its sequence reveals a divergent bacteriophytochrome with histidine kinase and cryptic response regulator motifs.

Abstract

The circadian oscillator of the cyanobacterium Synechococcus elongatus , like those in eukaryotes, is entrained by environmental cues. Inactivation of the gene cikA (circadian input kinase) shortens the circadian period of gene expression rhythms in S . elongatus by approximately 2 hours, changes the phasing of a subset of rhythms, and nearly abolishes resetting of phase by a pulse of darkness. The CikA protein sequence reveals that it is a divergent bacteriophytochrome with characteristic histidine protein kinase motifs and a cryptic response regulator motif. CikA is likely a key component of a pathway that provides environmental input to the circadian oscillator in S . elongatus .