Research On Coordination Mechanism Between Cell Cycle And Heterocyst Differentiation In Anabaena Sp.strain PCC7120

Cell cycle and cell differentiation are fundamental and important research topics in field of life science and medicine.Cell differentiation usually needs to escape from the cell cycle;"all cells arise only from pre-existing cells" summarizes the relationship between cell cycle and cell differentiation correctly.From last decades,a large number of genes and some aspects of these basic life processes have been found and analyzed,but the relationship and coordination mechanism between them haven't been explained in detail.Anabaena sp.strain PCC 7120 is a filamentous cyanobacterium,in response to deprivation of combined nitrogen in environment,vegetative cells differentiate specialized nitrogen-fixing cells called heterocysts.Heterocyst differentiation is one of the most ancient forms of cell differentiation.Exploring for better understanding of its mechanism has a special significance to understand basic life processes and their relationship.This study evaluated relationship between cell cycle and heterocyst differentiation,from perspective of DNA replication and repair,changes in cell morphology,distribution of DNA and photosynthetic pigment;subcellular localization of the DNA replication initiation protein DnaA.,DNA polymerase Pol I,DNA double-stranded damage response protein RecN,cytoskeleton protein MreB and cell division protein FtsZ and coordination mechanism between DNA replication,repair,in Anabaena sp.PCC7120.Results in this study shows that nucleoid of Anabaena was packaged independent.In the cell cycle of Anabaena,cytoplasm segregated before nucleoid.DnaA was usually located in the center of cell in hetR mutant,appeared at cell division plate during vegetative cell division.Pol I,expression was undulated along the filament and always co-located with chromosome in the cell.RecN formed a separate focal point randomLy within cells during the interphase of cell cycle,and moved to central division between daughter cells during vegetative cell division.MreB assembled areshaped line longitudinally located under cell membrane,and transformed to straight line through two daughter cells during cell division.FtsZ was formed Z ring in the cell division plate as in other bacteriums.In the process of heterocyst differentiation,cells at the end of filament were easier to form heterocysts;photosynthetic pigment degraded,chromosome formed an "X" or "Y" type structure first in proheterocyst,then presented zonal distribution in mature heterocyst because of the polar nodule.Normally there is a small amount of chromosomes at polar nodule with cyanophycin granules through cell membrane between vegetative cell and heterocyst.Pol I transcription increases up to two times in 6 hours of nitrogen deficiency,its transcription level in heterocyst was still higher than that of vegetative cell,but decline in extension of time.RecN appeared as a separate focal point in new heterocysts as in vegetative cells,then absent in mature heterocysts,and in hetR mutant cells,at the time of nitrogen deficiency,it was mostly co-localized with nucleoid-occupied regions in central part of cells.MreB location in heterocyst was the same as in vegetative cell.FtsZ formed Z ring in the middle of the proheterocyst;and disappeared in mature heterocyst.In addition,it was found that the DNA damage could inhibit the heterocyst differentiation,and induced DNA damage after heterocyst differentiation could causes abnormal heterocyst differentiation.In vegetative cell,DNA damage leads to an increase of the number of foci and location of RecN in cell center and in heterocyst,while it does not cause reappearance of RecN focal point.According to this research work,in Anabaena sp.PCC7120,it concluded that DNA replication and DNA repair systems are connected and cooperate with each other.Though,DNA restoration system in heterocyst was not complete.Heterocyst differentiation process,first need to start process of DNA replication and Z ring formation in cell division.Based on these results,heterocyst actually develops from immature cell of two daughter cells.