Hypersensitive Photic Responses and Intact Genome-Wide Transcriptional Control without the KaiC Phosphorylation Cycle in the Synechococcus Circadian System

Cyanobacteria are unique organisms with remarkably stable circadian oscillations. These are controlled by a network architecture that comprises two regulatory factors: posttranslational oscillation (PTO) and a transcription/translation feedback loop (TTFL). The clock proteins KaiA, KaiB, and KaiC are essential for the circadian rhythm of the unicellular speciesSynechococcus elongatus PCC 7942. Temperature-compensated autonomous cycling of KaiC phosphorylation has been proposed as the primary oscillator mechanism that maintains the circadian clock, even in the dark, and it controls genome-wide gene expression rhythms under continuous-light conditions (LL). However, thekaiC EE mutation (where “EE” represents the amino acid changes Ser431Glu and Thr432Glu), where phosphorylation cycling does not occurin vivo , has a damped but clearkaiBC expression rhythm with a long period. This suggests that there must be coupling between the robust PTO and the “slave” unstable TTFL. Here, we found that thekaiC EE mutant strain in LL was hypersensitive to the dark acclimation required for phase shifting. Twenty-three percent of the genes in thekaiC EE mutant strain exhibited genome-wide transcriptional rhythms with a period of 48 h in LL. The circadian phase distribution was also conserved significantly in most of the wild-type andkaiC EE mutant strain cycling genes, which suggests that the output mechanism was not damaged severely even in the absence of KaiC phosphorylation cycles. These results strongly suggest that the KaiC phosphorylation cycle is not essential for generating the genome-wide rhythm under light conditions, whereas it is important for appropriate circadian timing in the light and dark.