Circadian rhythms are endogenous biological timing processes that are ubiquitous in organisms ranging from cyanobacteria to humans. In the photoautotrophic unicellular cyanobacterium Synechococcus elongatus PCC 7942, under continuous light (LL) conditions, the transcription-translation feedback loop (TTFL) of KaiC generates a rhythmic change in the accumulation of KaiC relative to KaiA clock proteins (KaiC/KaiA ratio), which peak and trough at subjective dawn and dusk, respectively. However, the role of TTFL in the cyanobacterial circadian system remains unclear because it is not an essential requirement for the basic oscillation driven by the Kai-based posttranslational oscillator (PTO) and the transcriptional output mechanisms. Here, we show that TTFL is important for the circadian photic resetting property in Synechococcus. The robustness of PTO, which is exemplified by the amplitude of the KaiC phosphorylation cycle, changed depending on the KaiC/KaiA ratio, which was cyclic under LL. After cells were transferred from LL to the dark, the clock protein levels remained constant in the dark. When cells were transferred from LL to continuous dark at subjective dawn, the KaiC phosphorylation cycle was attenuated with a lower KaiC/KaiA ratio, a higher KaiC phosphorylation level, and a lower amplitude than that in cells transferred at subjective dusk. We also found that the greater the degree to which PTO was attenuated in continuous dark, the greater the phase shifts upon the subsequent light exposure. Based on these results, we propose that TTFL enhances resetting of the Kai-based PTO in Synechococcus.
