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NSF PR 98-46 - September 4, 1998
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GLOWING CYANOBACTERIA GIVES RESEARCHERS NEW CLUES
TO CIRCADIAN RHYTHMS
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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