Study On The Functional Mechanism Of An Iron Transport Protein CFTP During Acclimation Of Cyanobacteria To High Light Stress

Cyanobacteria are considered to be the first to provide oxygen by oxygenic photosynthesis necessary for starting the evolution of complex organisms on Earth.However,cyanobacteria have to adapt to an environment with reactive oxygen species(ROS)that cause damages to cells.To reduce the production of ROS,cyanobacteria need more iron to participate in the synthesis of Fe-S clusters and heme for a fast operation of linear electron transport chain.By constrast,cyanobacterial cells require a higher level of iron for synthesizing more Fe-S clusters and heme under high light conditions.Meanwhile,the subsequent oxygenation of the Earth's atmosphere by cyanobacteria causes a change that ferrous iron is oxidized by oxygen to ferric form,resulting in a fact that iron became a limiting nutrient in many aerobic habitats.Considering the prevalence of iron deficiency in nature and the high need of iron,we are wondering if cyanobacteria have to develop some oxygenic photosynthesis-specific(OPS)iron transport proteins to capture iron efficiently that is very important for cells survival under such conditions.However,little is known about these OPS iron transport proteins.In this thesis,we identified an OPS iron transport protein through screening a tagging library of Synechocystis sp.strain PCC 6803,and we named it as c FTP(cyanobacterial Fe transport protein).Main findings are as follows:(1)We screened and isolated two mutants that their growth is similar to the wild-type(WT)under low light but was retarded under high light compared with the WT through screening a tagging library of Synechocystis sp.strain PCC 6803 and using a screening strategy from low light to high light.To identify the genes inactivated by transposon tagging,we analyzed the sites of transposon insertion in both mutants.Sequencing analysis of the insertion region revealed that both mutants were tagged in a gene,cftp.(2)Through analyzing the various properties of c FTP,we found that c FTP is an OPS protein.We also found that the expression level of cftp is up-regulated under iron deficiency,and deletion of cftp decreased a total iron content,especially under high light conditions that need more iron.Furthermore,c FTP is located in the plasma membrane.Taking all these results together,we propose that c FTP is involved in iron transport.(3)We analyzed the accumulation levels of photosynthetic membrane protein complexes in WT and ?cftp mutant strains under different intensities of light by using blue native(BN)-PAGE followed by immunoblotting analyses.Our results indicated that except NDH-1,absence of c FTP had a specific effect on iron-contained photosynthetic complexes.This implied that c FTP can stabilize these photosynthetic membrane protein complexes under high light conditions,especially for the PSI;meanwhile,this is the reason why deletion of c FTP resulted in a slow growth phenotype of the mutant.(4)If the iron-dependent photosynthetic complexes were damaged,the activity of electron transport chain would be inevitably impaired.Through monitoring the rate of electron transport driven by PSII(ETRII)and the activity of whole electron transport chain,we found that c FTP plays an important role in maintaining the capacity of electron transport,thereby reducing the accumulation of ROS,being consistent with high light-sensitive phenotype of ?cftp mutant.In summary,this thesis identified an OPS protein——c FTP,which is located in the plasma membrane.Under high light conditions,c FTP facilitates a fast assembly of photosynthetic membrane protein complexes and consequently reduces the production of ROS by supplying enough iron into cells.These findings will increase our understanding on the cyanobacterial Fe transport proteins and the relationship between iron transport system and efficient photosynthesis.