Two circadian oscillators in one cyanobacterium

Organisms from all kingdoms of life have evolved diverse mechanisms to address the predictable environmental changes resulting from the Earth’s rotation. The circadian clock of cyanobacteria is a particularly simple and elegant example of a biological timing mechanism for predicting daily changes in the light environment. The three proteins KaiA, KaiB, and KaiC constitute the central timing mechanism that drives circadian oscillations in the cyanobacterium Synechococcus elongatus PCC 7942. In addition to the standard oscillator, Synechocystis sp. PCC 6803, another model organism for cyanobacterial research, harbors several divergent clock homologs. Here, we describe a potential new chimeric KaiA homolog that we named KaiA3. At the N-terminus, KaiA3 is similar to the NarL-type response regulator receiver domain. However, its similarity to canonical NarL transcription factors drastically decreases in the C-terminal domain, which resembles the circadian clock protein, KaiA. In line with this, we detected KaiA3-mediated stimulation of KaiC3 phosphorylation. Phosphorylation of KaiC3 was rhythmic over 48 h in vitro in the presence of KaiA3 and KaiB3 as well as in Synechocystis cells under free-running conditions after light/dark entrainment. This results in the presence of two different oscillators in a single-celled prokaryotic organism. Deletion of the kaiA3 gene leads to KaiC3 dephosphorylation and results in growth defects during mixotrophic growth and in the dark. In summary, we suggest that KaiA3 is a nonstandard KaiA homolog, thereby extending the KaiB3-KaiC3 system in Cyanobacteria and potentially other prokaryotes. ### Competing Interest Statement The authors have declared no competing interest.