Adaptive-response sensory kinase SasA

UniProtKB accession:  Q06904

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UniProtKB description:  Member of the two-component regulatory system SasA/RpaA involved in genome-wide circadian gene expression (PubMed:16882723). One of three clock output pathways. Participates in the KaiABC clock protein complex, which constitutes the main circadian regulator in cyanobacteria, via its interaction with KaiC. Required for robustness of the circadian rhythm of gene expression and involved in clock output (PubMed:10786837, PubMed:20133618, PubMed:34618577). KaiC enhances the autophosphorylation activity of SasA, which then transfers its phosphate group to RpaA to activate it. Phosphotransfer is maximal when KaiC phosphorylation is active during the circadian cycle; this two-component system is activated by fully phosphorylated KaiC (PubMed:16882723, PubMed:16707582, PubMed:23541768, PubMed:26113641, PubMed:34618577). A very robust clock is reconstituted with KaiA, KaiB, KaiC, SasA, CikA and RpaA; output is measured by transcription from an appropriate reporter (PubMed:34618577). In addition to its output function, recruits fold-shifted KaiB (KaiB(fs)) to KaiC to cooperatively form the KaiB(6):KaiC(6) complex (independent of SasA kinase activity); at physiological concentrations increases their association. At higher concentrations SasA and KaiB(fs) compete to bind to KaiC. Mutations that decrease cooperativity nearly phenocopy a deletion mutation (PubMed:34618577).