| FtsZ is the first protein identified to be located in the membrane of the prokaryotic cell division,and it is a homologous of tubulin in eukaryotes.FtsZ is the most conserved gene found in almost all prokaryotes.As the main cytoskeleton protein,it is necessary for cell division.Bacterial FtsZ has been extensively studied in vitro.FtsZ from Escherichia coli(EcFtsZ)was assembled into Single filaments with a length of 30-50 subunits,which showed a high GTPase activity.The cycles of polymerization,GTP hydrolysis and depolymerization make FtsZ filaments highly dynamic in vivo and in vitro.Together with a dozen other helper proteins,these fibrils form a highly dynamic contractile ring(Z-ring)in the body.The contractile ring is the main part of the bacterial cell splitter.At the end of bacterial division,the contractile ring contracts and divides the bacteria into two cells.Cyanobacteria,the simplest and most primitive single-celled prokaryotes,contain chlorophyll a and are the oldest organisms capable of oxygen-producing photosynthesis.Morphologically,cyanobacteria have the outer membrane characteristics of gram-negative bacteria.However,the regulatory system of cell division in cyanobacteria has the same specific components as that of gram-positive bacteria.The chloroplasts of plants are derived from endosymbiotic archaea cyanobacteria.Therefore,further study of cyanobacteria cell division will provide us more evidence to understand the bacterial cell division and the similarities and differences of division system between bacterial cyanbacterial and chloroplast will provide us new insights into the evolution of chloroplast division.Here,our study showed that Synechocystis sp.pcc 6803 FtsZ(SyFtsZ),in the presence of GTP,polymerized into large bundle filaments,which was similar to chloroplast FtsZ.Cyanobacteria have their unique biochemical properties.They not only form large straight bundles,but they also form large circular bundles of filaments.And the assembly of cyanobacteria FtsZ filaments showed two stages of assembly,Firstly,some short strand of the mixture,including single straight protofilaments,single curved filaments and the ring structure composed of multiple filaments.Next,large straight and circlar bundles were assembled.The diameter of the circlar bundles is about 200 to 300 nm.Sy FtsZ has a weak GTPase activity and a slow polymerization.Allignment of the sequences,we found two amino acids witin H8 helxi of Sy FtsZ were alanines,meanwhile,bacterial FtsZ were threonine or serine.Site-directed mutation from alanines to threonine and serine of Sy FtsZ increase its GTPase activity and reduce its bundle formation.We hypothesized that it may be related to the slow growth of cyanobacteria cells or the increase of the cell diameter.The large bundles structure may contribute to the stability of the FtsZ ring at the cell with a large diameter.The regulatory mechanism of cyanobacteria Min system on Z ring localization is different from that of gram-negative and gram-positive bacteria.The Min system of Synechocystis sp.pcc 6803 has a MinCDE system similar to that of gram-negative bacteria,as well as DivIVA similar protein Cdv3 of gram-positive bacteria.MinCD oscillation driven by Min E,And Cdv3 recruited MinC to the middle of the cell to complete the correct positioning of the Z ring,so as to ensure the correct division of cyanobacteria cells.Different from previous reports,the study found that the Synechocystis sp.pcc 6803 Min E could inhibit and disassemble the assembly of minc-mind copolymer,which was consistent with the role of Min E in bacteria and was a key component of the Min system.This study clarified the biochemical characteristics of FtsZ,a key cell division protein of cyanobacteria Synechocystis sp.pcc 6803,and provided new and powerful evidence for the regulation of Z ring localization by cyanobacteria Min system,which was of great significance for elucidation of its mechanism of chloroplast evolution. |
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