Circadian rhythms in rapidly dividing cyanobacteria

It was already known that circadian clocks typically require a stable cell cycle to maintain rhythmicity. However, uncertainty remained about whether these clocks could persist in cells dividing faster than the 24-hour cycle. Prior research has shown that circadian rhythms are often disrupted when cell division outpaces the clock's period. This gap motivated the investigation of whether cyanobacteria, which can divide rapidly, retain circadian regulation. The study aimed to clarify if the clock can function when cells divide multiple times per cycle. No prior work had resolved this contradiction between clock period and division rate. The question of how circadian timing persists in fast-dividing cells remained unresolved. This uncertainty drove the experimental approach to test clock function in Synechococcus under rapid growth conditions.

The researchers sought to determine if circadian rhythms can persist in cyanobacteria dividing faster than the typical 24-hour cycle. They focused on Synechococcus PCC 7942, a species known for its circadian clock. The study aimed to test the assumption that rapid cell division disrupts circadian function. The specific problem addressed was whether the clock can maintain rhythmicity when cells divide three or more times per cycle. This investigation was driven by the need to understand clock resilience in fast-growing cells. The motivation stemmed from unresolved questions about clock-cell cycle interactions. The study tested this hypothesis using bioluminescence and RNA profiling. The goal was to clarify if the clock functions under these conditions.

The team used a reporter strain of Synechococcus PCC 7942 to monitor bioluminescence as a clock output. They grew the bacteria under continuous exponential conditions with a 10-hour doubling time. Bioluminescence profiles were compared to a mathematical model of exponential cell growth and cosine oscillations. RNA abundance was measured during growth with a 5- to 6-hour doubling time. The model assumed each cell’s bioluminescence oscillated independently. The researchers tracked messenger RNA levels to detect circadian patterns. They tested whether the clock’s output remained rhythmic despite rapid division. The approach combined experimental data with computational modeling.

Bioluminescence in the reporter strain followed a cosine pattern matching the model predictions. This suggests the clock maintained rhythmicity even as cells divided rapidly. RNA levels for some genes showed circadian rhythms during exponential growth. The doubling time of 5 to 6 hours allowed cells to divide three times per cycle. Despite this, the clock’s output remained consistent with a 24-hour period. The data indicated that the clock can function in fast-dividing cells. The observed rhythms suggest the clock is not strictly tied to the cell cycle. These findings challenge the assumption that rapid division disrupts circadian function.

The cyanobacterial circadian clock functions in cells dividing multiple times per cycle. The study’s findings suggest that the clock can operate independently of the cell cycle. The observed bioluminescence and RNA rhythms support this conclusion. The data indicate that the clock does not require a stable cell cycle to maintain rhythmicity. The researchers propose that the clock’s mechanism allows it to persist despite rapid division. These results suggest the clock can function under conditions previously thought incompatible. The study’s implications suggest that circadian clocks may be more flexible than previously assumed. The findings provide evidence that the clock can maintain rhythms in fast-dividing cells.