Investigations Of Slr0320 Involved In High Light Acclimations Of Synechocystis Sp.PCC 6803

Light is not only a vital but potentially dangerous factor for photosynthesis and photosynthetic organisms.Upon exposure to high light(HL),several HL acclimations are triggered to alleviate photo-damage.When photo-damage exceeds the capacity of photo-repair,the imbalance between photo-damage and photo-repair occurs,and the photoinhibition becomes apparent,which would reduce the photosynthesis activity and survival of photosynthetic organisms.Thus,it is critical to investigate the HL acclimations for photosynthesis and photosynthetic organisms.Herein,we identified a HL-sensitive mutant?slr0320 in Synechocystis sp.PCC 6803(hereafter Synechocystis),by screening over 1000 different colonies of a transposon mutant library by comparison the growth under normal light(50?mol m^-2 s^-1,NL)and HL(220?mol m^-2 s^-1)conditions.And a complementary strain Ppet J::slr0320 in the background of the mutant?slr0320 was generated.The growth rate of the mutant?slr0320 was similar to WT and the complementary strain Ppet J::slr0320under NL but severely declined under HL.Net photosynthesis measured as oxygen evolution rate of HL grown?slr0320 cells was much lower,indicating that the lower net photosynthesis was the reason for the retarded growth of the mutant?slr0320 under HL.Immunodetection revealed that accumulation of major thylakoid proteins,such as subunits of photosystem I,photosystem II(PSII),cytochrome b₆f and the NDH complex,was similar between WT and the mutant?slr0320.BN-PAGE showed that the assembly of the PSII was unaltered in the mutant?slr0320.The chlorophyll fluorescence traces showed that the stable fluorescence under light(Fs)of the mutant?slr0320 was much higher than that of WT and the complementary strain Ppet J::slr0320,suggesting the higher portion of closed PSII centers in the mutant?slr0320.The kinetics of single flash-induced chlorophyll fluorescence increase and subsequent decay revealed the slower electron transfer from Q_A to Q_B in the mutant?slr0320 even under NL.The quantitative proteomics revealed that one of those low molecular weight(LMW)subunits of PSII,Psb L,was found to be significantly up-regulated in the?slr0320 mutant grown under NL.Previous reports have demonstrated that additional Psb L protein could optimize the binding of plastoquinone within the PSII complex and subsequent electron transfer.Therefore,we thought the enhanced expression of Psb L in the?slr0320 mutant under NL may partially restore the PSII electron transfer in the mutant under NL.The q RT-PCR(Quantitative Real-Time PCR)to investigate the transcripts of the genes encoding the other LMW subunits showed that transcription levels of psb H and psb I were significantly down-regulated in the?slr0320 mutant grown under HL.Many studies have demonstrated that these two gene products were highly related to the acceptor-side structure within PSII and were considered to regulate electron transfer from Q_A to Q_B.Therefore,we thought that the much more evidently retarded growth of the?slr0320 mutant under HL was possibly due to decreases in psb H and psb I.Sequence analysis showed that the Slr0320 protein contains two conserved domains:the B12-binding and the radical SAM domains,which resembled the class B radical SAM methyltransferases.Combining our data and previous reports of the radical SAM methyltransferases,a possibility that Slr0320 might be involved in methylation of plastoquinone,further affecting the HL acclimation of Synechocystis,cannot be excluded.Furthermore,based on quantitative proteomics data,the molecular mechanisms of the HL acclimation of Synechocystis were briefly discussed.