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Tick-Tock Goes a Bacterial Clock
by Lisa Seachrist Chiu
Researchers now better understand the "gears" on this unusual Circadian Clock.
Precise time-keeping proteins keep the biological clock ticking steadily.
Researchers in Erin K. O'Shea's laboratory at Harvard University lifted the face off an ancient bacterial clock and revealed how a rugged and reliable timekeeping mechanism keeps the clock ticking out daily cycles for weeks in a test tube.
The team discovered the mechanism relied on the sequential addition and subtraction of phosphate groups to one of three proteins constituting the clock.
Their study builds on dogma-smashing work by Takao Kondo and colleagues at Nagoya University in Japan. Conventional wisdom held that all circadian clocks—cellular mechanisms that regulate physiological activity on a roughly 24-hour schedule—kept time via a protein regulating its own gene expression. Kondo's group discovered that the circadian clock governing the ancient, photosynthesizing bacteria known as cyanobacteria involves no genes and just three proteins, and can function outside the cell fueled only by a phosphate source called ATP.
While the Japanese group knew that the clock ticked by adding and removing phosphate molecules from the KaiC protein with the aid of two protein “gears”—KaiA and KaiB—they didn't know how the addition and subtraction kept time.
O'Shea's group revealed a cycle where, with the help of KaiA, two sites on the KaiC protein gained phosphate groups in sequence: first one site, then the other. Once both sites on KaiC have phosphate groups, KaiB alerts KaiA, triggering phosphate removal in the same sequence. Over the course of a day, KaiC slowly cycles from no phosphate groups to two phosphate groups, back to one phosphate group, finally ending up with no phosphate groups once again. Details of the work were published October 4, 2007, in Science Express.
All circadian clocks make adjustments in response to changes in light, O'Shea says. For cyanobacteria—the blue-green algae responsible for 70 percent of the Earth's photosynthesis—the clock may allow them to anticipate daylight and rev up the production of proteins needed for photosynthesis. The question is how. “Other laboratories have identified some of the proteins involved,” says O'Shea. “Our goal is to understand how the inputs from the environment feed into this clock.”
Photo: Bert Meyers / Photo Researchers, Inc.