bims-ciryme 2026-03-01 papers

Curr Biol. 2026 Feb 25. pii: S0960-9822(26)00138-7. [Epub ahead of print]

The free-running property of circadian clocks is needed to tolerate changing photoperiods.

Andrew F Schober, Caroline Holmes, Stephanie Palmer, Michael J Rust.

A defining feature of circadian rhythms is an internal oscillator that is self-sustaining in constant conditions, but the utility of this free-running property is not clear.1 In cyanobacteria, two alternative timing systems are found: a canonical circadian clock and an hourglass-like system incapable of free-running.2 By swapping genetic elements, we engineer a pathway to convert a circadian clock into an hourglass. We show that the performance of these systems is similar in a balanced light-dark cycle, but the hourglass shows dysregulated transcription in long photoperiod days and fails to provide resistance to midday UV exposure. A minimal mathematical model shows that inability to adapt to a changing photoperiod is a generic limitation of these hourglass systems that free-running clocks can overcome. We conclude that the ability to occupy niches far from the equator, where daylength is highly variable, demands a self-sustaining circadian rhythm, consistent with the observed geographical range of cyanobacterial species.3 Our work establishes a genetic model system to study how environmental pressures led to the evolution of circadian rhythms.

Keywords: cyanobacteria, circadian, seasonality, mathematical modeling

DOI: https://doi.org/10.1016/j.cub.2026.01.066

Sci Rep. 2026 Feb 24.

Effects of sodium chloride on circadian period and temperature compensation of KaiC phosphorylation.

Eugene Kim, Makayla Adams, Soren Tyree, Yongick Kim.

The circadian clock is an internal timekeeping system that generates ~ 24-h cycles in physiology and behavior, maintaining a remarkably consistent period across physiological conditions and temperature1. Sodium chloride (NaCl) is a key physiological ion whose concentration varies across species, yet its influence on circadian rhythms remains poorly understood. Using a reconstituted cyanobacterial oscillator composed of KaiA, KaiB, and KaiC, we investigated how NaCl modulates circadian period and temperature compensation. Increasing NaCl concentrations progressively shortened the circadian period without substantially affecting oscillation amplitude, mimicking the effects of KaiB titration. While temperature compensation was maintained across varying KaiB concentrations, changes in NaCl partially disrupted temperature compensation, as Q₁₀ values correlated positively with salt concentration. We propose that NaCl perturbs the equilibrium of KaiB conformations and oligomerization, normally stabilized across physiological temperatures, thereby modulating circadian period and temperature compensation. These findings provide insight into how physiological salt levels influence circadian timing and drive the diversification of clock proteins across species.

DOI: https://doi.org/10.1038/s41598-026-40224-z

Science. 2026 Feb 26. 391(6788): eady2822

A cellular basis for the mammalian nocturnal-diurnal switch.

Early mammals were nocturnal while dinosaurs dominated the daytime. Mammalian transition to daytime activity accelerated after the Cretaceous-Paleogene extinction, but the underlying mechanisms remain unclear. We identified a conserved cell-intrinsic, thermodynamic mechanism that likely facilitated this shift. In cells from diurnal mammals, protein synthesis, phosphorylation, and circadian timing were less sensitive to temperature changes than were cells from nocturnal mammals. Comparative genomics revealed accelerated evolution within essential signaling pathways, including mechanistic target of rapamycin (mTOR), that increase the robustness of diurnal cellular clocks to thermal and osmotic perturbation. In nocturnal mice, mTOR inhibition shifted cells, tissues, and behavior toward diurnal activity. These findings uncover a genetic and biochemical basis for nocturnal-diurnal switching, emphasizing how cellular signaling networks can encode complex phenotypes such as temporal niche selection.

DOI: https://doi.org/10.1126/science.ady2822