A novel mutation in kaiC affects resetting of the cyanobacterial circadian clock

Light is the most important factor controlling circadian systems in response to day-night cycles. In order to better understand the regulation of circadian rhythms by light in Synechococcus elongatus PCC 7942, we screened for mutants with defective phase shifting in response to dark pulses. Using a 5-h dark-pulse protocol, we identified a mutation in kaiC that we termed pr1, for phase response 1. In the pr1 mutant, a 5-h dark pulse failed to shift the phase of the circadian rhythm, while the same pulse caused a 10-h phase shift in wild-type cells. The rhythm in accumulation of KaiC was abolished in the pr1 mutant, and the rhythmicity of KaiC phosphorylation was reduced. Additionally, the pr1 mutant was defective in mediating the feedback inhibition of kaiBC. Finally, overexpression of mutant KaiC led to a reduced phase shift compared to that for wild-type KaiC. Thus, KaiC appears to play a role in resetting the cellular clock in addition to its documented role in the feedback regulation of circadian rhythms.