Interactions Between Histidine Kinase NblS and the Response Regulators RpaB and SrrA are Involved in the Bleaching Process of the Cyanobacterium Synechococcus elongatus PCC 7942

Cyanobacteria have developed a light-harvesting antenna complex known as the phycobilisome. When cells are starved for nutrients or exposed to high light, the phycobilisome is rapidly degraded (bleaching). It has been suggested that in the cyanobacterium Synechococcus elongatus PCC 7942, the bleaching process is regulated by a two-component histidine kinase, NblS. To clarify the signaling pathway involving NblS, we identified the NblS-interacting response regulators RpaB and SrrA. In vitro assays using recombinant proteins showed that both RpaB and SrrA can receive phosphoryl groups from autophosphorylated NblS; the NblS-interacting protein SipA clearly enhances the phosphotransfer activity from NblS to RpaB and SrrA. In addition, NblS prefers SrrA over RpaB as the phosphotransfer target with or without SipA. Gel mobility shift assay revealed that both RpaB and SrrA can bind to the upstream region of nblA, a major regulatory factor in the bleaching process. nblA transcript accumulates in nblS or rpaB mutants even under normal growth conditions, while in the srrA disruptant the nblA transcripts are slightly up-regulated under stress conditions. These observations suggest that the bleaching signal transduction pathway via NblS is regulated by RpaB and that SrrA is partially involved.