Assembly and disassembly dynamics of the cyanobacterial periodosome.

In vitro incubation of three Kai proteins, KaiA, KaiB, and KaiC, with ATP induces a KaiC phosphorylation cycle that is a potential circadian clock pacemaker in cyanobacterium Synechococcus elongatus PCC 7942. The Kai proteins assemble into large heteromultimeric complexes (periodosome) to effect a robust oscillation of KaiC phosphorylation. Here, we report real-time measurements of the assembly/disassembly dynamics of the Kai periodosome by using small-angle X-ray scattering and determination of the low-resolution shapes of the KaiA:KaiC and KaiB:KaiC complexes. Most previously identified period-affecting mutations could be mapped to the association interfaces of our complex models. Our results suggest that the assembly/disassembly processes are crucial for phase entrainment in the early synchronizing stage but are passively driven by the phosphorylation status of KaiC in the late oscillatory stage. The Kai periodosome is assembled in such a way that KaiA and KaiB are recruited to a C-terminal region of KaiC in a phosphorylation-dependent manner.