Two KaiABC systems control circadian oscillations in one cyanobacterium

Area of Science:

• Microbiology
• Molecular Biology
• Chronobiology

Background:

• Cyanobacteria utilize circadian clocks to anticipate daily environmental changes.
• A canonical KaiA-KaiB-KaiC oscillator is common, but diverse homologs exist with unknown functions.
• Synechocystis sp. PCC 6803 contains additional KaiB and KaiC homologs (KaiB2, KaiC2, KaiB3, KaiC3).

Purpose of the Study:

• To identify and characterize novel Kai protein homologs in Synechocystis sp. PCC 6803.
• To investigate the function of the KaiA3-KaiB3-KaiC3 system in circadian rhythms.
• To determine the interplay between the canonical and alternative circadian oscillators.

Main Methods:

• Identification of a chimeric KaiA homolog (KaiA3) with a unique domain structure.
• In vitro phosphorylation assays using KaiA3, KaiB3, and KaiC3 to demonstrate circadian oscillations.
• Genetic manipulation (mutations and deletions) of kaiA3 and kaiAB1C1 gene clusters.
• Analysis of cellular backscattering rhythms to assess circadian period and amplitude.

Main Results:

• A novel KaiA3 protein, with a response-regulator receiver domain and KaiA-like C-terminus, was identified.
• The KaiA3-KaiB3-KaiC3 system exhibits circadian phosphorylation oscillations in vitro.
• Mutations in kaiA3 impair KaiC3 phosphorylation and cause growth defects.
• Both KaiA1B1C1 and KaiA3B3C3 oscillators are essential for sustained, high-amplitude 24-hour cellular rhythms.

Conclusions:

• Synechocystis sp. PCC 6803 possesses at least two interconnected circadian oscillators.
• The KaiA3-KaiB3-KaiC3 system represents a functional alternative or complementary circadian clock.
• These interconnected oscillators are vital for maintaining robust circadian behavior and cellular fitness.