Sugar Time

Metabolic activity, not light, drives the circadian clock in cyanobacteria.

By | March 1, 2016

SWEET TIMEKEEPERS: S. elongatus cyanobacteria (chlorophyll visible here) set their circadian clocks by metabolism rather than light.YI LIAO, UNIVERSITY OF CHICAGO

EDITOR'S CHOICE IN CELL & MOLECULAR BIOLOGY

The paper
G.K. Pattanayak et al., “Controlling the cyanobacterial clock by synthetically rewiring metabolism,” Cell Reports, 13:2362-67, 2015.

Tiny tickers
Like many organisms, the photosynthetic cyanobacterium Synechococcus elongatus has a circadian clock that cycles with light/dark rhythms. The timekeeper has just three core proteins—KaiA, KaiB, and KaiC—making it the simplest circadian clock known to science. Given that researchers have failed to identify light-sensitive clock components in S. elongatus, but have demonstrated that Kai proteins respond to metabolic activity, some scientists suggest that cyanobacteria are synchronizing to the downstream metabolites of photosynthesis, as opposed to light itself.

Decoupling metabolism
To tease apart light-dark cycles and metabolism, researchers led by Michael Rust at the University of Chicago engineered a strain of S. elongatus that could grow without photosynthesis by metabolizing externally supplied glucose.

Sugar dependency
The team found that these cyanobacteria’s clock phases were essentially unresponsive to light cues in the presence of glucose, and that in constant darkness, they could be entrained by a periodic glucose supply. The results establish metabolic activity as the primary clock driver in S. elongatus. “All you need is a metabolic cycle and the [clock] proteins will follow it,” says Rust.

A general mechanism?
The discovery of this role for metabolism in some of the world’s oldest organisms indicates that “probably the most ancient thing that led to the creation of circadian clocks was the presence of daily metabolic cycling,” Rust explains. As for gut bacteria, which also show daily rhythms in the absence of light-dark cycles, the Weizmann Institute’s Eran Elinav, who was not involved in the work, says the study “brings the very attractive hypothesis that metabolic activity or different combinations of metabolites may drive periodicity in this system as well.”

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