Sciences researchers have proposed that circadian clocks may additionally provide an "intrinsic" adaptive value [6, 7].

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Prokaryotic cyanobacteria express robust circadian (daily) rhythms under the control of a clock system that appears to be similar to those of eukaryotes in many ways. On the other hand, the KaiABC-based core cyanobacterial clockwork is clearly different from the transcriptiontranslation feedback loop model of eukaryotic clocks in that the cyanobacterial clock system regulates gene expression patterns globally, and specific clock gene promoters are not essential in mediating the circadian feedback loop. A novel model, the oscilloid model, proposes that the KaiABC oscillator ultimately mediates rhythmic changes in the status of the cyanobacterial chromosome, and these topological changes underlie the global rhythms of transcription. The authors suggest that this model represents one of several possible modes of regulating gene expression by circadian clocks, even those of eukaryotes. Keywords circadian; biological clock; kai; prokaryote; global gene expression; supercoiling Animals, plants, fungi, and cyanobacteria all display daily or circadian rhythms in their biochemistry, physiology, and/or behavior that are controlled by biological clocks (Dunlap et al., 2004). A variety of biological processes in various organisms are controlled by biological clocks such as gene expression, photosynthesis, sleeping/waking, and development, and this regulation is thought to help organisms adapt to the daily changes in light, temperature, and other factors in their environment. Circadian regulation of gene expression (i.e., the levels of specific proteins in cells) can be accomplished by clock control of transcription, mRNA stability, translation, and protein degradation. A particularly fascinating example of translational control by a circadian clock is that in the dinoflagellate alga, Gonyaulax (Rossini et al., 2003). However, we focus our discussion specifically on clock control of cyanobacterial gene expression at the level of transcription.In general, studies of circadian gene expression in eukaryotes have often used microarray analyses to show that a relatively small proportion of genes (~5%-15%) in eukaryotic genomes display rhythms in mRNA abundance. While microarray technology is well advanced, it should be noted that microarrays are not very sensitive to small changes of mRNA abundance, and moreover, they are not a good measure of rhythmic transcriptional activity for mRNAs that are either very unstable or very stable. Therefore, microarray results might not be reflective of transcriptional control in detail.

Circadian clocks are found in a wide variety of organisms from cyanobacteria to mammals. Many believe that the circadian clock system evolved as an adaption to the daily cycles in light and temperature driven by the rotation of the earth. Studies on the cyanobacterium, Synechococcus elongatus PCC 7942, have confirmed that the circadian clock in resonance with environmental cycles confers an adaptive advantage to cyanobacterial strains with different clock properties when grown in competition under light-dark cycles. The results thus far suggest that in a cyclic environment, the cyanobacterial strains whose free running periods are closest to the environmental period are the most fit and the strains lacking a functional circadian clock are at a competitive disadvantage relative to strains with a functional clock. In contrast, the circadian system provides little or no advantage to cyanobacteria grown in competition in constant light. To explain the potential mechanism of this clock-mediated enhancement in fitness in cyanobacteria, several models have been proposed; these include the limiting resource model, the diffusible inhibitor model and the cell-to-cell communication model. None of these models have been excluded by the currently available experimental data and the mechanistic basis of clock-mediated fitness enhancement remains elusive.

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