Tuning the circadian period of cyanobacteria up to 6.6 days by the single amino acid substitutions in KaiC

Significance Circadian clocks, biological oscillators with temperature-compensated periods close to 24 h, enhance fitness in Earth’s daily cycle. Although many circadian period mutants have helped to elucidate the circadian clock mechanism, the tuning mechanism of 24-h period remains unclear. In this study, we found that mutants at position 402 of cyanobacterial clock protein KaiC alter circadian periods dramatically, from 0.6 to 6.6 d. Despite the wide-range tunability, temperature compensation of period was kept nearly intact. As these astounding results can be observed in in vitro system, residue 402 is the important position for tuning mechanism of period. Furthermore, we found that the volume of residue 402 of KaiC is the key factor in determination of the length of circadian period.