Hypersensitive photic responses and intact genome-wide transcriptional control without the KaiC phosphorylation cycle in the Synechococcus circadian system

Miki Umetani, Norimune Hosokawa, Yohko Kitayama, Hideo Iwasaki

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    Cyanobacteria are unique organisms with remarkably stable circadian oscillations. These are controlled by a network architecture that comprises two regulatory factors: posttranslational oscillation (PTO) and a transcription/translation feedback loop (TTFL). The clock proteins KaiA, KaiB, and KaiC are essential for the circadian rhythm of the unicellular species Synechococcus elongatus PCC 7942. Temperature-compensated autonomous cycling of KaiC phosphorylation has been proposed as the primary oscillator mechanism that maintains the circadian clock, even in the dark, and it controls genome-wide gene expression rhythms under continuous-light conditions (LL). However, the kaiCEE mutation (where "EE" represents the amino acid changes Ser431Glu and Thr432Glu), where phosphorylation cycling does not occur in vivo, has a damped but clear kaiBC expression rhythm with a long period. This suggests that there must be coupling between the robust PTO and the "slave" unstable TTFL. Here, we found that the kaiCEE mutant strain in LL was hypersensitive to the dark acclimation required for phase shifting. Twenty-three percent of the genes in the kaiCEE mutant strain exhibited genome-wide transcriptional rhythms with a period of 48 h in LL. The circadian phase distribution was also conserved significantly in most of the wild-type and kaiCEE mutant strain cycling genes, which suggests that the output mechanism was not damaged severely even in the absence of KaiC phosphorylation cycles. These results strongly suggest that the KaiC phosphorylation cycle is not essential for generating the genome-wide rhythm under light conditions, whereas it is important for appropriate circadian timing in the light and dark.

    Original languageEnglish
    Pages (from-to)548-555
    Number of pages8
    JournalJournal of Bacteriology
    Issue number3
    Publication statusPublished - 2014 Feb


    ASJC Scopus subject areas

    • Microbiology
    • Molecular Biology

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