CikA, a Bacteriophytochrome That Resets the Cyanobacterial Circadian Clock

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.

The pseudo‐receiver domain of CikA regulates the cyanobacterial circadian input pathway

“…Circadian phenotypes were monitored by the oscillation in bioluminescence from a kaiB::luxAB reporter strain, which tracks circadian expression from the kaiBC promoter. A cikA null strain exhibits a shortened circadian period (approximately 23 h rather than WT 25 h, Table 1 and Schmitz et al ., 2000), a reduced amplitude of oscillation (Fig. 1C), a failure to reset the phase (relative timing of peak expression) after an environmental stimulus (Schmitz et al ., 2000) (Fig.…”

“…A cikA null strain exhibits a shortened circadian period (approximately 23 h rather than WT 25 h, Table 1 and Schmitz et al ., 2000), a reduced amplitude of oscillation (Fig. 1C), a failure to reset the phase (relative timing of peak expression) after an environmental stimulus (Schmitz et al ., 2000) (Fig. 2), and an elongated cell morphology (Miyagishima et al ., 2005).…”

“…The cikA gene was first identified from a Tn 5 transposon insertion mutant that showed a subtle defect in light-responsive regulation of photosynthesis genes; subsequently, the mutation was shown to have a more striking effect on circadian rhythms of gene expression (Schmitz et al ., 2000). A cikA null strain still exhibits circadian rhythmicity as monitored by bioluminescence produced from luciferase reporter genes; however, the circadian period is shortened by about 2 h, the amplitude of oscillation is greatly reduced, and, diagnostic of an input pathway defect, the ability to sense a 5 h pulse of darkness that resets the phase of circadian rhythms in the wild-type (WT) strain is almost completely abolished (Schmitz et al ., 2000).…”

“…The cikA gene was first identified from a Tn 5 transposon insertion mutant that showed a subtle defect in light-responsive regulation of photosynthesis genes; subsequently, the mutation was shown to have a more striking effect on circadian rhythms of gene expression (Schmitz et al ., 2000). A cikA null strain still exhibits circadian rhythmicity as monitored by bioluminescence produced from luciferase reporter genes; however, the circadian period is shortened by about 2 h, the amplitude of oscillation is greatly reduced, and, diagnostic of an input pathway defect, the ability to sense a 5 h pulse of darkness that resets the phase of circadian rhythms in the wild-type (WT) strain is almost completely abolished (Schmitz et al ., 2000). Other factors that affect environmental sensing by the clock have been identified (Kutsuna et al ., 1998;Ivleva et al ., 2005), but none has as great an effect as CikA on the ability to reset the clock other than a component of the circadian oscillator itself (Kiyohara et al ., 2005); the oscillator is presumably the target of the input pathway (Williams et al ., 2002).…”

Biological Rhythms Workshop IA: Molecular Basis of Rhythms Generation

“…Resetting the phase of the rhythm in response to abrupt external stimuli, such as pulses of darkness or temperature, occur through the circadian input kinase (CikA) protein (Schmitz et al 2000). Accumulation of CikA occurs during the night (Ivleva et al 2006), and low levels of CikA protein are present at higher light intensities.…”