A day in the life of a cyanobacterium
Most organisms exhibit daily cycles that are driven by endogenous circadian clocks. Until recently, transcription/translation feedback on central clock genes has been proposed as the core mechanism of circadian rhythm generation in any organism. A similar model was proposed for the functions of three essential clock genes, kaiA, kaiB and kaiC, in the cyanobacterium Synechococcus elongatus PCC 7942 (hereinafter, Synechococcus) under continuous light conditions. The model proposed the importance of feedback regulation in which KaiC inhibits its own (kaiBC) transcription, while being enhanced by KaiA. However, the circadian rhythm of KaiC phosphorylation persisted for at least three cycles after the rapid disappearance of kaiABC mRNA under continuous dark conditions even in the presence of excess transcription and translation inhibitors.
The Synechococcus genome seems to be primarily regulated by light/dark cycles and is dramatically modified by the protein-based circadian oscillator. Although bacterial transcription has been considered to correlate well with proteomic profiles, this work suggests that there is a much greater discrepancy between the two profiles than previously thought.
Cyanobacterial daily life with Kai-based circadian and diurnal genome-wide transcriptional control in Synechococcus elongatus. PNAS USA July 30, 2009 doi: 10.1073/pnas.0902587106
In the unicellular cyanobacterium Synechococcus elongatus PCC 7942, essentially all promoter activities are under the control of the circadian clock under continuous light (LL) conditions. Here, we used high-density oligonucleotide arrays to explore comprehensive profiles of genome-wide Synechococcus gene expression in wild-type, kaiABC-null, and kaiC-overexpressor strains under LL and continuous dark (DD) conditions. In the wild-type strains, >30% of transcripts oscillated significantly in a circadian fashion, peaking at subjective dawn and dusk. Such circadian control was severely attenuated in kaiABC-null strains. Although it has been proposed that KaiC globally represses gene expression, our analysis revealed that dawn-expressed genes were up-regulated by kaiC-overexpression so that the clock was arrested at subjective dawn. Transfer of cells to DD conditions from LL immediately suppressed expression of most of the genes, while the clock kept even time in the absence of transcriptional feedback. Thus, the Synechococcus genome seems to be primarily regulated by light/dark cycles and is dramatically modified by the protein-based circadian oscillator.
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Tags: Bacteria, Biology, cyanobacteria, Genetics, Microbiology, Science
