An Evolutionary Conserved Multi-Stress Sensory Histidine Kinase NblS Associates With Photosystem II Proteins And Responds To Its Redox Status In The Cyanobacterium Synechococcus elongatus PCC 7942

Responding to stress caused by various environmental changes is essential for living organisms. In cyanobacteria that perform oxygenic photosynthesis, the highly conserved histidine kinase Hik33/NblS homologs respond to diverse stressors such as high light, low temperature, high salts, high osmolarity, and reactive oxygen species. However, how this unique protein kinase responds to such divergent stresses remains unknown. This study has focused on the underlying stress sensing mechanism of NblS in Synechococcus elongatus PCC 7942. First, the sensory response by NblS was analyzed in vivo by monitoring the NblS-regulated hliA transcript accumulation with treatment of various benzoquinone reagents known as photosystem II (PSII) electron acceptors. It was found that molecular responses induced by various stresses were diminished in the presence of 2,6-dichloro-1,4-benzoquinone, which accepts electrons specifically from the PSII-bound plastoquinone Q _B . Cell fractionation analysis indicated that NblS was localized in the thylakoid membrane, which was consistent with its predicted membrane-spanning structure. In the thylakoid membrane, NblS was found in approximately 400 kDa and 800 kDa unknown complexes in clear native PAGE (CN-PAGE). Immunoprecipitation analysis of the cross-linked thylakoid membrane revealed that NblS is associated with D2 and CP47 proteins but not with CP43 protein, and thus it was suggested that dimeric NblS is associated with RC47-like complex, an assembly intermediate complex of PSII, as [RC47like-NblS ₂ ] (370 kDa) or [RC47like-NblS ₂ ] ₂ (740 kDa). We propose that the redox status of an RC47-bound plastoquinone molecule is a cue for the NblS response.