Texas A&M Researchers Reset Bacterial Clocks

Jet lag is the most obvious evidence that people have an internal timing device capable of telling time without cues from the environment, says Susan Golden, a biologist at Texas A&M University. Only when your clock resets itself after a few days can you sleep and wake at the right times.

Humans aren't the only organisms with these internal, or circadian, clocks. Plants and animals also have internal timing devices and about 10 years ago, scientists were stunned to learn that simple, single-celled bacteria had biological clocks as well. Even more surprising, says Golden, was the realization that the bacterial clock does everything the clocks of its more complicated plant and animal counterparts do.

In the Aug. 4 issue of the journal Science, Golden's research group reports their identification of a gene - called the cikA gene - that codes for a protein important in resetting the circadian clock in a species of cyanobacteria.

The Texas A&M researchers mutated bacteria by introducing a transposon, or a jumping gene, into the bacteria. This jumping gene inserted itself randomly into a cell, yielding a collection of randomly mutated bacteria, each of which produces its own mutant colony. The researchers tested each colony, looking for a defect in the bacterial clock and the inserted transposon served as a tag, indicating which gene was mutated.

The cyanobacteria were genetically engineered to give off light, or bioluminescence, using a luciferase gene that codes for the enzyme that produces light. By measuring the amount of light given off by the bacteria, the researchers indirectly measured the expression of a gene controlled by the clock. A plot of the amount of light given off by the normal bacteria shows a peak of light emission every 24 hours, followed 12 hours later by a trough, when little or no light was given off.

The researchers found that if they varied the time when the bacteria with the mutant cikA gene saw light and dark, the mutant bacteria did not recognize that environmental cue and continued peaking and troughing as though they had not seen a change in their schedules. The mutated bacteria could not reset their clocks and also thought a day was two hours shorter than it actually is.

These facts, says Golden, provide strong evidence that the gene and the protein it codes for play a significant role in resetting the bacterium's circadian clock.

An organism's ability to reset its circadian clock is important, says Golden, because day length changes during the year, thereby changing the environment so that sunrise and sunset are at different times each day.

"By having an internal clock that is somewhat flexible," says Golden, "you can go to different locations and have your clock synchronized with your environment. And even staying in one location, you can have your clock synchronized appropriately at different times of the year."