Unlocking Circadian Rhythms: Timekeeping Breakthroughs

Almost all living things have an internal 24-hour clock which remains accurate regardless of temperature or other environmental changes. This clock is a highly sophisticated, yet simple, timekeeping mechanism that is critically important to many functions, including metabolism and survival. Until now, understanding the influences keeping the internal clock ticking reliability was unknown.

However, in a recent study published in PNAS, researchers from The University of Osaka have revealed that circadian clock oscillation in cyanobacteria is controlled by factors intrinsic to one of the proteins that controls it, in a manner that is unaffected by environmental conditions.

Even the smallest, photosynthetic organisms have internal clocks, including cyanobacteria. These microorganisms are vital for aquatic environments, agriculture, and biotechnology. Given their vitality, it is even more important to ensure the correct timing of biological processes for photosynthesis during the day, and respiration at night.

Cyanobacteria are known to possess the simplest known circadian clock, involving only three primary proteins: KaiA, B, and C. It was these proteins that were the focus of the investigation.

"Though the cyanobacterial circadian clock is very simple, and can be reconstructed with three proteins, we still wanted to understand how these simple elements work together," says lead author, Kumiko Ito-Miwa. "It is critical to understand how the reliability of the circadian rhythm is maintained under different environmental conditions, as it affects an incredibly wide variety of cellular processes."

To do this, the researchers examined more than 20 mutations in the KaiC clock protein, with disturbed clock periods ranging from 15 to 60 hours. Through this, they were able to demonstrate that the circadian clock could maintain accurate timekeeping both in vitro and in vivo, regardless of environmental changes, through properties inherent to the clock proteins. This included the activity of ATPase, an enzyme responsible for producing chemical energy, which allows cells to perform their duties in various processes.

"The activity of this protein, which acts as the pacemaker of the cyanobacterial clock, did not change in response to different environmental conditions. This property, which appears to be innate to the protein itself, is likely critical for preserving circadian timing despite environmental changes," explains Kumiko Ito-Miwa, lead author, building on a concept originally proposed and long pursued by Takao Kondo.

The findings suggest that the environment inside cyanobacterial cells may fine-tune the circadian clock to align it with Earth's 24-hour cycle, offering significant insight into the fundamental question of how living organisms measure time.

Fig. 1

Caption: Rhythms of KaiC period mutants in cells and in vitro (representative examples).

Even when temperature or light intensity changes, the circadian period of the wild type (normal strain), as well as those of short- and long-period mutants, remains largely unchanged. Moreover, compared with the in vitro clock, the intracellular clock shifts slightly toward the Earth's 24-hour cycle: short-period mutants lengthen slightly, whereas long-period mutants shorten slightly.

Credit: Kumiko Ito-Miwa