We're all familiar with the use of William Paley's famous
watch analogy as an indicator of design. But did you know
that cyanobacteria have their own watches? In the 9/4/99
issue of Science, an article appears entitled "New Timepiece
Has a Familiar Riing," written by Marcia Barinaga. The article
even has a nice picture of cyanobacteria, with a caption that
some might enjoy:
"Timeless design: Cyanobacteria such as this one use clocks
that work much like ours."
Barinaga begins the article as follows:
"Pendulums, quartz crystals, oscillating atoms - human beings
have invented many different ways to keep track of time. Mother
Nature, however, seems early on to have hit on one good design
for the molecular clocks that govern circadian rhythms."
The article then discusses the work of Takao Kondo's work, who
has been the pioneer in characterizing the molecular basis of
circadium rhythms in cyanobacteria (the simplest example, BTW,
of such a rhythm).
For some time, biologists have noted that cyanobacteria seem
to obey an internal clock which paces 24-hour cycles of various
biological activities. The clock that calibrates these events
is in essence a genetic oscillator involving at least three gene
products - kaiA, kaiB, and kaiC.
It is also true that other organisms have a circadian rhythm
employing clock proteins, but the cyanobacterial proteins
are very different, leading biologists to propose that these
clocks arose independently.
KaiABC appear to involve an irreducibly complex system.
Remove any one of the genes, and the clock stops working.
Replace the right gene after it has been removed, and the
clock starts up again. Other proteins are probably involved
in this clock function, but have yet to be identified.
How does the clock work? The evidence supports this
model: Early in the day, the kai genes are transcribed
and translated. Then, kaiA protein turns up the expression
of the kaiB and C genes. Then, after a delay, kaiC protein
turns off the gene expression of all three. Kai protein levels fall
(probably as a function of the cell's protein degradation systems),
thus as kaiC proteins levels fall, the genes all get expressed again.
And the 24-hour cycle then repeats. This, as Kondo says, is a
"feedback-loop model for the circadian oscillator" of
cyanobacteria.
It should be stressed that this system looks like nothing other
than a clock. When you remove the three genes, cyanobacteria
can still exist in the lab (although long term studies have not
been done); they simply lack their circadian rhythm.
Furthermore, as far as I can tell, these proteins have no homologs.
Thus, it appears right now that cyanobacteria have a unique molecular
clock that is somehow used by other cellular systems to coordinate
their activity.
Mike
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