Abstract
Proteins KaiA, KaiB and KaiC constitute a biochemical circadian oscillator in the cyanobacterium Synechococcus elongatus. It has been reported kaiA inactivation completely abolishes circadian oscillations. However, we show here that kaiBC promoter activity exhibits a damped, low-amplitude oscillation with a period of approximately 24 h in kaiA-inactivated strains. The damped rhythm resonates with external cycles with a period of 24–26 h, indicating that its natural frequency is similar to that of the circadian clock. Double-mutation experiments reveal that kaiC, kaiB, and sasA (encoding a KaiC-binding histidine kinase) are all required for the damped oscillation. Further analysis suggests that the kaiA-less damped transcriptional rhythm requires KaiB-KaiC complex formation and the transcription-translation feedback loop, but not the KaiC phosphorylation cycle. Our results provide insights into mechanisms that could potentially underlie the diurnal/circadian behaviors observed in other bacterial species that possess kaiB and kaiC homologues but lack a kaiA homologue.
Highlights
Proteins KaiA, KaiB and KaiC constitute a biochemical circadian oscillator in the cyanobacterium Synechococcus elongatus
In the KaiABCbased oscillator, we demonstrated that the in vitro-reconstituted circadian oscillator monitored by the KaiC phosphorylation cycle switched to a damped oscillator at low temperatures, and that the amplitude of the damped oscillation is resonated with forced a b
We suggest that KaiB–KaiC complex formation is important for the generation and period determination of the damped oscillation in the absence of KaiA
Summary
Proteins KaiA, KaiB and KaiC constitute a biochemical circadian oscillator in the cyanobacterium Synechococcus elongatus. Consistent with the proposed role of KaiA, kaiA-lacking species fail to exhibit oscillation under continuous conditions[11,12], whereas they display diurnal variations in transcription and cellcycle control under light–dark (LD) cycles.[13,14,15] the diurnal but not free-running rhythm in nitrogen fixation has been reported in even non-cyanobacterial purple bacterium, Rhodopseudomonas palustris, which harbors kaiB and kaiC homologs without kaiA16. Differing from KaiC in Synechococcus, KaiA is not essential for enhancing the basal autophosphorylation activity of the KaiC homolog in Prochlorococcus MED4 and Rhodopseudomonas[12,16] In both species, KaiC homologs undergo phosphorylation in the light and dephosphorylation in the dark[16,17].
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