The cyanobacterial circadian clock, the only rigorously established prokaryotic circadian system, is driven by the KaiABC core oscillator and accompanied by dynamic changes in KaiA and KaiC localization. Because KaiB had not previously been imaged in living cells, the spatial organization of the complete core oscillator remained unresolved. Recent work by H. J. Bevir, C. C. Hooper, P. Saikley, T. Chaljian, et al. (Microbiol Spectr 14:e01845-25, 2026, https://doi.org/10.1128/spectrum.01845-25) shows that KaiB localizes to the cell poles at night, completing the spatial model of the cyanobacterial clock. Using a frankenbody-based imaging strategy in bacteria for the first time, the authors overcome longstanding barriers to visualizing KaiB in vivo. They also demonstrate that the RNA-binding protein Rbp2 co-localizes with KaiB and KaiC, and that its RNA-binding activity is required for robust localization. These results support a model in which RNA-mediated scaffolding assembles a nighttime polar clock complex, providing substantial advancement in understanding clock mechanism and emphasizing the importance of spatial organization in circadian robustness.
