Bacterial circadian rhythms

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. The conclusion that only eukaryotes have circadian oscillators seemed reasonable, because it was assumed that an endogenous timekeeper with a period close to 24 hours would not be useful to prokaryotic organisms that often divide more rapidly than once every 24 hours. The assumption might be stated as, "why have a timer for a cycle that is longer than your lifetime?" While intuitive, the conclusion was flawed. It was based on the assumption that a bacterial cell is equivalent to a sexually reproducing multicellular organism. However, a bacterial culture is more like a mass of protoplasm that grows larger and larger and incidentally subdivides. From this perspective, it is reasonable that a 24-hour temporal program could be adaptive to a rapidly dividing protoplasm if the fitness of that protoplasm changes as a function of daily alterations in the environment (light intensity, temperature, etc.).

In 1985–86, 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, who were the first to demonstrate temperature compensation of a daily rhythm in the marine cyanobacterium, Synechococcus WH7803.

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