Department of Electrical Engineering and Biological Science, Waseda University, Shinjuku, Tokyo 162-8480, Japan.
Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15396-401. doi: 10.1073/pnas.1019612108. Epub 2011 Sep 6.
Circadian rhythms are a fundamental property of most organisms, from cyanobacteria to humans. In the unicellular obligately photoautotrophic cyanobacterium Synechococcus elongatus PCC 7942, essentially all promoter activities are controlled by the KaiABC-based clock under continuous light conditions. When Synechococcus cells are transferred from the light to continuous dark (DD) conditions, the expression of most genes, including the clock genes kaiA and kaiBC, is rapidly down-regulated, whereas the KaiC phosphorylation cycle persists. Therefore, we speculated that the posttranslational oscillator might not drive the transcriptional circadian output without de novo expression of the kai genes. Here we show that the cyanobacterial clock regulates the transcriptional output even in the dark. The expression of a subset of genes in the genomes of cells grown in the dark was dramatically affected by kaiABC nullification, and the magnitude of dark induction was dependent on the time at which the cells were transferred from the light to the dark. Moreover, under DD conditions, the expression of some dark-induced gene transcripts exhibited temperature-compensated damped oscillations, which were nullified in kaiABC-null strains and were affected by a kaiC period mutation. These results indicate that the Kai protein-based posttranslational oscillator can drive the circadian transcriptional output even without the de novo expression of the clock genes.
昼夜节律是大多数生物体的基本特性，从蓝藻到人都是如此。在单细胞需氧自养蓝藻集胞藻 PCC 7942 中，在连续光照条件下，基本上所有启动子的活性都受基于 KaiABC 的时钟控制。当集胞藻细胞从光照条件转移到连续黑暗（DD）条件时，大多数基因的表达，包括时钟基因 kaiA 和 kaiBC，迅速下调，而 KaiC 的磷酸化循环仍然存在。因此，我们推测，在没有 kai 基因的从头表达的情况下，翻译后振荡器可能不会驱动转录的昼夜节律输出。在这里，我们表明，蓝藻生物钟即使在黑暗中也能调节转录输出。在黑暗中生长的细胞基因组中一组基因的表达受到 kaiABC 缺失的显著影响，而且黑暗诱导的幅度取决于细胞从光照到黑暗的转移时间。此外，在 DD 条件下，一些黑暗诱导基因转录本的表达表现出温度补偿的阻尼振荡，在 kaiABC 缺失菌株中这种振荡被消除，并且受 kaiC 周期突变的影响。这些结果表明，基于 Kai 蛋白的翻译后振荡器即使没有时钟基因的从头表达，也可以驱动昼夜节律的转录输出。
