A KaiC-associating SasA-RpaA two-component regulatory system as a major circadian timing mediator in cyanobacteria

Naoki Takai, Masato Nakajima, Tokitaka Oyama, Ryotaku Kito, Chieko Sugita, Mamoru Sugita, Takao Kondo, Hideo Iwasaki*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

169 Citations (Scopus)


KaiA, KaiB, and KaiC clock proteins from cyanobacteria and ATP are sufficient to reconstitute the KaiC phosphorylation rhythm in vitro, whereas almost all gene promoters are under the control of the circadian clock. The mechanism by which the KaiC phosphorylation cycle drives global transcription rhythms is unknown. Here, we report that RpaA, a potential DNA-binding protein that acts as a cognate response regulator of the KaiC-interacting kinase SasA, mediates between KaiC phosphorylation and global transcription rhythms. Circadian transcription was severely attenuated in sasA (Synechococcus adaptive sensor A)- and rpaA (regulator of phycobilisome-associated)-mutant cells, and the phosphotransfer activity from SasA to RpaA changed dramatically depending on the circadian state of a coexisting Kai protein complex in vitro. We propose a model in which the SasA-RpaA two-component system mediates time signals from the enzymatic oscillator to drive genome-wide transcription rhythms in cyanobacteria. Moreover, our results indicate the presence of secondary output pathways from the clock to transcription control, suggesting that multiple pathways ensure a genome-wide circadian system.

Original languageEnglish
Pages (from-to)12109-12114
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number32
Publication statusPublished - 2006 Aug 8


  • Biological clock
  • Phosphorelay
  • Response regulator
  • Synechococcus

ASJC Scopus subject areas

  • General


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