Cyanobacterial clock proteins
In molecular biology, the cyanobacterial clock proteins are the main circadian regulator in cyanobacteria. The cyanobacterial clock proteins comprise three proteins: KaiA, KaiB and KaiC. The kaiABC complex may act as a promoter-nonspecific transcription regulator that represses transcription, possibly by acting on the state of chromosome compaction. This complex is expressed from a KaiABC operon.
| KaiA domain | |||||||||
|---|---|---|---|---|---|---|---|---|---|
crystal structure of circadian clock protein kaia from synechococcus elongatus | |||||||||
| Identifiers | |||||||||
| Symbol | KaiA | ||||||||
| Pfam | PF07688 | ||||||||
| InterPro | IPR011648 | ||||||||
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| KaiB domain | |||||||||
|---|---|---|---|---|---|---|---|---|---|
solution structure of the n-terminal domain of synechococcus elongatus sasa (average minimized structure) | |||||||||
| Identifiers | |||||||||
| Symbol | KaiB | ||||||||
| Pfam | PF07689 | ||||||||
| Pfam clan | CL0172 | ||||||||
| InterPro | IPR011649 | ||||||||
| CDD | cd02978 | ||||||||
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| KaiC | |||||||||
|---|---|---|---|---|---|---|---|---|---|
crystal structure of full length circadian clock protein kaic with phosphorylation sites | |||||||||
| Identifiers | |||||||||
| Symbol | KaiC | ||||||||
| Pfam | PF06745 | ||||||||
| Pfam clan | CL0023 | ||||||||
| InterPro | IPR014774 | ||||||||
| CDD | cd01124 | ||||||||
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See also: bacterial circadian rhythms
In the complex, KaiA enhances the phosphorylation status of kaiC. In contrast, the presence of kaiB in the complex decreases the phosphorylation status of kaiC, suggesting that kaiB acts by antagonising the interaction between kaiA and kaiC. The activity of KaiA activates kaiBC expression, while KaiC represses it.
Also in the KaiC family is RadA/Sms, a highly conserved eubacterial protein that shares sequence similarity with both RecA strand transferase and lon protease. The RadA/Sms family are probable ATP-dependent proteases involved in both DNA repair and degradation of proteins, peptides, glycopeptides. They are classified in as non-peptidase homologues and unassigned peptidases in MEROPS peptidase family S16 (lon protease family, clan SJ). RadA/Sms is involved in recombination and recombinational repair, most likely involving the stabilisation or processing of branched DNA molecules or blocked replication forks because of its genetic redundancy with RecG and RuvABC.