PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Circadian rhythm of the facultative cyanobacterium Plectonema boryanum in the dark. Kazuki Terauchi^*,1, Mitsunori Katayama¹, Yuichi Fujita², Takao Kondo¹, ¹ Graduate School of Science, Nagoya University, Nagoya, Japan² Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Japan

ABSTRACT- Cyanobacteria are the simplest organisms to exhibit circadian rhythms. We have studied the mechanisms of circadian rhythms using the cyanobacterium Synechococcus elongatus PCC 7942 and demonstrated that kaiABC gene cluster is essential for the clock functions. The obligate photoautotrophic organism S. elongatus PCC 7942 is not a suitable model to address the questions that photosynthesis is involved in sustaining the expression of circadian rhythms and that light is necessary for the oscillation of the circadian clock. To address the questions, we tried to monitor gene expression in the facultative cyanobacterium Plectonema boryanum which exhibits heterotrophic growth using glucose in complete darkness. A promoterless segment of luciferase genes was introduced downstream of the promoter for the P. boryanum chlB gene, which encodes a subunit B of light-independent protochlorophyllide reductase, or petF gene encoding ferredoxin. These reporter constructions were introduced in P. boryanum. We monitored the bioluminescence of reporter strains by an automated monitoring system as promoter activity. Bioluminescence of the reporter strains oscillated with a period of about 24 h in continuous light. In addition, we cloned kaiABC genes from P. boryanum, showing that a circadian clock gene cluster kaiABC was conserved as well as other cyanobacteria. These data indicates that this organism has the same mechanism controlling circadian rhythms as S. elongatus PCC 7942. When bioluminescence was monitored in continuous dark, it oscillated with a period about 21 h. This result suggests that photosynthetic metabolism is not necessary for the oscillation of the circadian clock to persist and that light signal is needed for the clock to cycle in circadian periodicity. The results of the effect of DCMU, an inhibitor of the photosystem II activity, on the oscillation of bioluminescence will be discussed.

KEY WORDS: kaiABC, circadian rhythm, cyanobacteria, Plectonema boryanum