Role of KaiC phosphorylation in the circadian clock system of Synechococcus elongatus PCC 7942

Sep 7, 2004Proceedings of the National Academy of Sciences of the United States of America

How KaiC protein changes affect the daily biological clock in Synechococcus elongatus bacteria

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Abstract

Phosphorylation at two specific sites on KaiC is essential for maintaining the circadian rhythm in Synechococcus elongatus PCC 7942.

KaiC serves as a negative regulator of circadian gene expression.
Two autophosphorylation sites, Ser-431 and Thr-432, were identified as critical for KaiC function.
Phosphorylation at these sites is required for KaiC to maintain circadian oscillation.
Single mutants showed reduced phosphorylation, while the double mutant completely lacked phosphorylation.
Nonphosphorylatable KaiC could not form a complex with other clock proteins in the cells.

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In the cyanobacterium Synechococcus elongatus PCC 7942, KaiA, KaiB, and KaiC are essential proteins for the generation of a circadian rhythm. KaiC is proposed as a negative regulator of the circadian expression of all genes in the genome, and its phosphorylation is regulated positively by KaiA and negatively by KaiB and shows a circadian rhythm in vivo. To study the functions of KaiC phosphorylation in the circadian clock system, we identified two autophosphorylation sites, Ser-431 and Thr-432, by using mass spectrometry (MS). We generated Synechococcus mutants in which these residues were substituted for alanine by using site-directed mutagenesis. Phosphorylation of KaiC was reduced in the single mutants and was completely abolished in the double mutant, indicating that KaiC is also phosphorylated at these sites in vivo. These mutants lost circadian rhythm, indicating that phosphorylation at each of the two sites is essential for the control of the circadian oscillation. Although the nonphosphorylatable mutant KaiC was able to form a hexamer in vitro, it failed to form a clock protein complex with KaiA, KaiB, and SasA in the Synechococcus cells. When nonphosphorylatable KaiC was overexpressed, the kaiBC promoter activity was only transiently repressed. These results suggest that KaiC phosphorylation regulates its transcriptional repression activity by controlling its binding affinity for other clock proteins.

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Role of KaiC phosphorylation in the circadian clock system of Synechococcus elongatus PCC 7942

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