Humans and other organisms that live for days or years have countless 24-hour cycles of chemical and biological activity. You'd think these circadian rhythms would be absent in bacteria that pass through two or more generations in a matter of hours. In a big surprise reported in today's issue of Science, however, bacteria that may never survive a sunset appear to have circadian rhythms, implying that circadian clocks may be a nearly universal feature of life on Earth.
It was "an absence of evidence, rather than evidence of absence" that persuaded most researchers that biological clocks don't exist in organisms whose fleeting lives amount to less than a day, says Susan Golden, a molecular biologist at Texas A&M University and a report co-author. To remedy that absence, Golden's lab, along with plant physiologist Takao Kondo and colleagues at Nagoya University in Japan, developed an easy-to-read gauge of changing photosynthetic activity in colonies of the cyanobacterium Synechococcus, a blue-green alga whose one-celled organisms divide as often as once every 5 to 6 hours.
The researchers engineered a strain in which a Synechococcus gene normally turned on during photosynthesis signals its activity by switching on the gene for luciferase, an enzyme from another bacterium that drives a light-emitting chemical reaction. When they exposed the transgenic strain to light and temperatures that foster rapid growth, the luminescent glow from the colonies showed cyclical changes that repeated every 24 hours. Because several generations of cells live and die within that period, the 24-hour clock was somehow passed from each cell to its progeny, without being reset. Golden speculates that this timepiece "tells photosynthetic cells when to get their biochemistry geared up for dawn, whether they're of generation one or generation four."
"The results are profound, for a number of different reasons," says Steven Kay, a geneticist at the Scripps Research Institute in La Jolla, California, who studies circadian clocks in plants. "Most intriguing," he says, the find suggests the existence of a regulatory mechanism that isn't perturbed by cell division. Kay says the study also "raises the possibility that many other bacteria maintain circadian clocks. "Perhaps people should go and look for them," he says.