Study On The Relationship Between ApcE-Arm2 And PBSs Assembly,State Transition Of Synechocystis Sp.Pcc6803

State transition is an important photoprotection mechanism to maintain excitation energy balance between photosystems（PSs）in response to fluctuating light environment rapidly and this mechanism is induced by oxidizing or reducing plastoquinone（PQ）pool.In plants and green algae,this change is sensed by cytochrome b₆f（Cyt b₆f）,interacting with a specific kinase to trigger light harvesting complex II（LHC II）movement.In contrast,although the state transition of cyanobacteria has been extensively studied,the molecular mechanisms involved in phycobilisomes（PBSs）have not yet been arrived at a conclusion.Synechocystis sp.PCC6803（Synechocystis）is the most widely existing type of PBSs.It had been shown that the deletion of Apc D and the PB-loop of ApcE inhibited state transition in previous studies.Synechocystis was used as the study object,and wild-type（WT）was used as the template to construct mutant strains missing all or part of ApcE-Arm2domain in this study.On the one hand,PBSs of Arm2-related mutants was extracted by sucrose density gradient supercentrifugation method and the effect of deletion of these domains on PBSs assembly was studied by biochemistry and spectroscopy.On the other hand,the modulated pulse fluorimeter-2500（PAM-2500）was used to detect the state transition.The results are as follows:（1）Missing Arm2 domain and Arm2（6-36）domain completely suppressed state transitions.Missing Arm2（37-67）and Arm2（99-129）domain did not affect state transitions.State transition could be directly attributed to the destruction of terminal energy emitter ApcE,which affected the assembly of PBSs and indirectly attributed to the interference of energy transfer pathway from PBSs to PSs.（2）There were no intact PBSs missing Arm2 and Arm2（6-36）domain.The energy transfer from terminal energy emitters to PSs was blocked,indicating that PBSs assembly was affected.Combined with the previous studies,it could be further concluded that missing Arm2 domain affected the assembly of basal（B）cylinders.The higher C_CPC/C_APCratio ofΔ（arm2（37-67））in 0.5-0.75 M zone indicated that missing Arm2（37-67）domain affected the connection between PBSs and rods.Δ（arm2（99-129））had the same sucrose density zones,protein polypeptide compositions and similar spectral characteristics as WT.Missing Arm2（99-129）had no effect on PBSs assembly and energy transfer.Combined the molecular interactions within PBSs with the phenotype of state transition and supposed the role of between PBSs and membrane,between disc and disc interaction of the B cylinders in this physiological process,it provided further evidence for the"PBSs movement"model or"energy spillover"model or"combine PBSs movement with energy spillover"model and a new idea for the study of the molecular level involved in state transition in this study.