Three genes essential to circadian rhythms in bacteria, the simplest organisms known to have such "internal clocks," have been identified by scientists.

The research 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, of the United State's National Science Foundation which funded the research.

To identify genes involved in circadian rhythms, the researchers used a gene for a glowing enzyme to indicate the activity of another gene that they knew the circadian clock controlled.

Whenever the circadian clock was working, the cell made luminescent chemicals causing the cell to glow with a predictable pattern throughout the day.

Once they could spot bacteria 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 theorise, are integral components of the feedback loop that drives the circadian clock.

The researchers do not believe 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.