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Concept
Bacterial circadian rhythm
Summary
Bacterial circadian rhythms, like other circadian rhythms, are endogenous "biological clocks" that have the following three characteristics: (a) in constant conditions (i.e. constant temperature and either constant light {LL} or constant darkness {DD}) they oscillate with a period that is close to, but not exactly, 24 hours in duration, (b) this "free-running" rhythm is temperature compensated, and (c) the rhythm will entrain to an appropriate environmental cycle.
Until the mid-1980s, it was thought that only eukaryotic cells had circadian rhythms. It is now known that cyanobacteria (a phylum of photosynthetic eubacteria) have well-documented circadian rhythms that meet all the criteria of bona fide circadian rhythms. In these bacteria, three key proteins whose structures have been determined can form a molecular clockwork that orchestrates global gene expression. This system enhances the fitness of cyanobacteria in rhythmic environments.
Before the mid-1980s, it was believed that only eukaryotes had circadian systems.
In 1985–6, several research groups discovered that cyanobacteria display daily rhythms of nitrogen fixation in both light/dark (LD) cycles and in constant light. The group of Huang and co-workers was the first to recognize clearly that the cyanobacterium Synechococcus sp. RF-1 was exhibiting circadian rhythms, and in a series of publications beginning in 1986 demonstrated all three of the salient characteristics of circadian rhythms described above in the same organism, the unicellular freshwater Synechococcus sp. RF-1. Another ground-breaking study was that of Sweeney and Borgese.
Inspired by the research of the aforementioned pioneers, the cyanobacterium Synechococcus elongatus was genetically transformed with a luciferase reporter that allowed rhythmic gene expression to be assayed non-invasively as rhythmically "glowing" cells. This system allowed an exquisitely precise circadian rhythm of luminescence to be measured from cell populations and even from single cyanobacterial cells.
Official source
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