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Journal of Biological Rhythms
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Expression of the Circadian Clock-Related Gene pex in Cyanobacteria Increases in Darkness and Is Required to Delay the Clock

Naoki Takai

International Graduate School of Arts and Sciences, Yokohama City University, Seto 22-2, Kanazawa-ku, Yokohama 236-0027, Japan

Shingo Ikeuchi

International Graduate School of Arts and Sciences, Yokohama City University, Seto 22-2, Kanazawa-ku, Yokohama 236-0027, Japan

Katsushi Manabe

International Graduate School of Arts and Sciences, Yokohama City University, Seto 22-2, Kanazawa-ku, Yokohama 236-0027, Japan

Shinsuke Kutsuna

International Graduate School of Arts and Sciences, Yokohama City University, Seto 22-2, Kanazawa-ku, Yokohama 236-0027, Japan, kutsuna{at}yokohama-cu.ac.jp

The time measurement system of the unicellular cyanobacterium Synechococcus elongatus PCC 7942 is analogous to the circadian clock of eukaryotic cells. Circadian clock-related genes have been identified in this strain. The clock-related gene pex is thought to maintain the normal clock period because constitutive transcription or deficiency of this gene causes respectively longer (~28 h) or shorter (~24 h) circadian periods than that of the wild type (~25 h). Here, the authors report other properties of pex in the circadian system. Levels of pex mRNA increased significantly in a 12-h exposure to darkness. Western blotting with a GST-Pex antibody revealed a 13.5-kDa protein band in wild-type cells that were incubated in the dark, while this protein was not detected in pex-deficient mutant cells. Therefore, the molecular weight of the Pex protein appears to be 13.5 kDa in vivo. The PadR domain, which is conserved among DNA-binding transcription factors in lactobacilli, was found in Pex. In the pex mutant, several 12-h light/12-h dark cycles reset the phase of the clock by 3 h earlier (phase advance) compared to wild-type cells. The degree of the advance in the pex mutant was proportional to the number of exposed light-dark cycles. In addition, ectopic induction of pex with an inducible Escherichia coli promoter, Ptrc, delayed the phase in the examined recombinant cells by 2.5 h (phase delay) compared to control cells. These results suggest that the dark-responsive gene expression of pex delays the circadian clock under daily light-dark cycles.

Key Words: circadian clock • cyanobacteria • input • pex • padR • phase delay

Journal of Biological Rhythms, Vol. 21, No. 4, 235-244 (2006)
DOI: 10.1177/0748730406289400
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