| TA system is composed of antitoxin and toxin genes. The toxin, encoded by the toxingene, is an stable protein, has different biochemical activities. The toxin control theimportant physiological activities of cells by acting on pecific targets which can resulting incell growth arrest or cell death. The antitoxin, encoded by the antitoxin gene, is an unstableprotein due to its susceptibility to ATP-dependant protease.The toxin is able to beneutralized by the cognate antitoxin through the toxin-antitoxin complex. The TA complexcan autoregulate the transcription of TA system by binding to DNA fragment in the operonvia the antitoxin. There are several hypotheses for the physiological functions of TA systemsabundant on the bacterial chromosome. The current results of studies for TA systems showthat some chromosomal TA systems play important roles in bacterial response to variousenvironmental stresses and allow bacterial adaptation to changeable environments.There areat least31different species of TA system familyon the chromosome of SynechocystisPCC6803. But the physiological, biochemical and genetic function of thesesystems is notclear. Therefore, the study on the TA systems in cyanobacterium has an importantsiganificance to interpret its physiological, biochemical and genetic function and reveal themolecular mechanism of cyanobacterium adaptation to environmental stresses..Here, westudied the relBE homologous gene relBE1as follows:（1）To determine the transcriptional regulation of the TA systems in E. coli cells, theregulatory cyanobacterial strains containing the lacZ transcriptionally fused to relBE1wereconstructed; To determine the interaction between the toxin and the antitoxin, relBE1wasco-expressed in E.coli BL21(DE3), the induced products were co-purified by usingaffinity-capture technique.（2）To analyse the degradation of toxin and antitoxin in the presence of the proteaseLons or ClpP2s/Xs, selection-expression system was used.（3）To investigate the physiological function of TA system expression products, themutant strain with relBE1deletion was constructed by the gene-knockout technique, and thephenotype of the constructed mutant was analyzed under stressful conditions. Thecomplementary experiment was also done; The relBE1over expression mutants wasconstructed. To know the role of cell expression on adaption to environmental stress, thegrowth phenotypes of the over expression mutant strains under normal culture conditions and stress conditions was analyzed.（4）To determine the transcriptional regulation of the TA system and its codingproduct under different growth conditions, LacZ activities of the transcriptional andregulatory mutants under normal culture conditions and the specific stress conditions wereanalyzed.The results and conclusions of this study are followed：（1）In heterologous host cells, the promoter of relBE1have transcriptional activity.But unlike the regulation profiles of other reported TA systems, the RelB1antitoxin is ableto positively regulate the transcription of relBE1TA system; Antitoxin RelB1and the toxinRelE1could form the TA complex through interaction with each other.（2）Both proteases Lons and ClpP2s/Xs were able to degrade the antitoxins RelB1and thus activated relBE1TA system.（3）The growthof relBE1deletion mutant was significantly inhibited under cadmiumstress; Incomplementary experiments, the deletion mutant could restore the growthphenotype under cadmium stress condition. The system components can improve thetolerance to cadmium of Synechocystis cells which can indicates that the relBE1systemparticipates in cyanobacteria to cadmium stress.（4）The results of the relBE1transcriptional regulation function in the natural hostcells was consistent with the results in E.coli cells. The relB1expression products couldactivate its operon transcription, and strengthened the activation function under Cd stresscondition.The level of transcription of relBE1system was increased under Cd Stress... |
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