The cycle of day and night is one of the most recurrent and predictable environmental change on our planet. Consequently, organisms evolved mechanisms that give them an ability to measure time over 24 hours and allow them to anticipate and prepare for periodic changes in the availability of light. These mechanisms are known as circadian clocks and they govern rhythmic changes in gene activity and protein levels driving daily oscillations in metabolism, physiology and behavior of organisms across all domains of life.
Time information encoded in the circadian clock of the photosynthetic bacterium Synechococcus elongatus is transmitted to the transcription factor RpaA; in turn, RpaA drives circadian fluctuations in gene activity. Deletion of rpaA disrupts gene expression rhythms and renders cells unable to survive the night, which is a time of energy limitation for photosynthetic bacteria that depend on sunlight to thrive. This defect in growth of the rpaA mutant cells suggests an important role for circadian regulation of physiology in conditions when light and dark periods alternate.
To date, detailed understanding of mechanisms by which the circadian system brings about competitive advantages to an organism has been missing. In our work we used the known viability defect of the rpaA mutant cells in light/dark cycling conditions as a gateway to learn how the circadian program schedules important physiological processes to give an organism an ability to better survive in a periodically changing environment.
We found that, while wild type cells prepare for darkness by storing sugars during the day and activating expression of sugar burning enzymes at dusk to liberate energy reserves during the night, the rpaA mutant cells are unable to make these preparations. This inability to make metabolic preparations for darkness leads to a defect in the maintenance of internal energy levels at night in the rpaA mutant cells. Provision of an external sugar source combined with restoration of the defective sugar metabolism pathways rescues the ability of the rpaA mutant cells to maintain energy in the dark and to grow in light/dark cycles. Our results demonstrate that one physiological role of the circadian system in S. elongatus is to coordinate sugar reserve formation during the day with carbon utilizing metabolic pathways activated at dusk.
In our work we uncover many additional genes that are not activated at dusk in the rpaA mutant cells. In the future it will be interesting to study whether other pathways important for cell growth and survival are defective in these cells.