Glowing Cyanobacteria Gives Researchers New Clues to Circadian Rhythms
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Three genes essential to circadian rhythms in cyanobacteria, the simplest organisms known to have such "internal clocks," have been identified by scientists funded by the National Science Foundation (NSF). The research, by biologists Carl Johnson of Vanderbilt University and Susan Golden of Texas A & M University, is published in this week's issue of Science.
"Circadian rhythms enable organisms to react to the two most predictable events on Earth -- day and night," said Shil DasSarma, program manager in NSF's division of molecular and cellular biosciences, which funded the research. The clocks that power circadian rhythms are complex mechanisms of chemical reactions that control the timing of events in cells.
To identify genes involved in the circadian rhythm process, the researchers used a gene for a bioluminescent enzyme to indicate the activity of another gene that they knew the circadian clock controlled. Whenever the circadian clock was working, the cell made bioluminescent proteins rhythmically-causing the cell to glow with a predictable pattern throughout the day. This made it easier for researchers to identify which cyanobacteria had working circadian clocks, since they were the ones glowing like fireflies.
Once they could spot cyanobacteria without such clocks, or whose clocks did not keep the correct time, the researchers could find which genes were not functioning properly. What they found was a cluster of three genes, which they named kaiABC, after the Japanese word for cycle, "kai". KaiABC contains the information that the cell will use to make proteins called KaiA, B and C, respectively. The Kai proteins, they theorize, are integral components of the feedback loop that drives the circadian clock.
"The expression of KaiC is critically important for setting the phase of the clock," said Golden. The researchers found that the levels of kaiC gene expression increase during daytime and decrease during nighttime. But an overabundance of KaiC protein through either period can measurably shift the timing of the clock. Adding too much KaiC while the amount of the protein is naturally rising, daytime, causes the clock to advance. Whereas too much KaiC when the levels ought to be falling, nighttime, causes the clock to slow. Altogether, high levels of KaiC protein can leave the cell in a state of perpetual twilight.
The researchers do not believe, however, that the Kai feedback mechanism can account for the entire 24-hour period of the clock. Even so, a single mutation in any of the Kai genes can alter, or even halt, the timing of the clock.
The kai genes do not resemble those that have been previously seen in the circadian workings of mammals and fruit flies. But Johnson and Golden believe that the basic workings that power the circadian clocks of cyanobacteria may have features in common to all clocks.
"Outlining the mechanisms in the simplest creatures known to have a circadian clock is likely to impact our thinking about how all clocks, even ours, function," said Johnson. "From cyanobacteria, we can picture how the circadian rhythms first evolved-when bacteria first learned the time of day."
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