Characterization Of Site-2-protease Slr1821 In The Ammonium Stress Response Of Synechocystis Sp. PCC6803

Organisms adapt to the environmental stress,mainly through the perception of environmental stress and the transduction of the signaling finally to the gene transcription to regulate the expression of the responding gene.They synthesize the corresponding proteins or enzymes,which participate in metabolic regulation to maintain the balance.The regulation of sigma factor by site-2-protease is a conservative transmembrane signal transduction mechanism,and it is involved in the acclimation of environmental stress.Synechocysts sp.PCC6803 is a model organism for studying environmental stress.It genome contains genes for four S2Ps: Sll0862,Slr0643,Sll0528,Slr1821.Three of them have been proved to be important in a variety of stress responses.But so far,no studies have been reported on Slr1821.In this paper,slr1821 gene was completely knocked out to construct the mutant strain ?slr1821.The mutant was characterized physiologically as comparing with the wild type under various environmental stresses,especially the ammonium stress.The transcriptome analysis of wild-type and ?slr1821 mutant under ammonium stress was compared to determine the main role of Slr1821 in response to ammonium stress.The following conclusions were obtained.?1?It was found that the phenotype of ?slr1821 mutant was similar with the phenotype of wild-type under high light stress,glucose mixotrophic growth,acid stress,salt stress and ethanol stress.Thus it is speculated that Slr1821 protease does not play a critical role in these stresses response or its function can be compensated by other S2 P proteins.?2?Slr1821 protease plays a very important role in the recovery of Synechocysts sp.PCC6803 after heat stress.The deletion of slr1821 resulted in the defect of recovery after 44?heat treatment.The phycobiliprotein and chlorophyll could not restore after heat treatment in mutant,as well the photosystem II and photosystem I were subjected to a certain degree of damage.?3?Under 15 mM?NH₄?₂SO₄,mutant could not grow at all,indicating that Slr1821 played an important role in the acclimation to ammonium stress.The photosynthetic active center?mainly PSII?was impaired in mutant.The NH4+ assimilation was impeded as well.Thus,it was indicated that Slr1821 is involved in the repair of photosynthetic system and the regulation of ammonia assimilation.Its absence resulted in the impaired photosynthetic system and the weakening of ammonium assimilation,thus hindering the growth of mutant.?4?Through transcriptomic sequencing and analysis,it was found that the group II ?-factor,the other S2 P protease,the metabolic pathways of protein synthesis,photosynthesis system,carbon fixation and nitrogen metabolism are involved in the acclimation to ammonium stress in Synechocysts sp.PCC6803.Slr1821 played a role in these metabolic pathways.?5?Further analysis of the differential expression genes between mutant and wild type,or before and after 15 m M?NH₄?₂SO₄ revealed interesting results.Under 15 mM?NH₄?₂SO₄,wild-type downregulated the genes involved in nitrogen transportation as well as ammonium assimilation pathways,such as glnN?slr0288?and glnA?slr1756?.While such downregulation was not observed in ?slr1821 mutant,indicating that Slr1821 was involved in the regulation of nitrogen transport and ammonium assimilation.Data digging showed that two upregulated gene,ssl1911 and sll1104 may be involved in the assimilation of ammonium,which may belong to the pathways of known or novel ammonium transformation pathway.The abcance of the upregulation of these two genes in ?slr1821 mutant thus put them under the regulation of Slr1821.