Role Of NtcB Gene In The Regulation Of Carbon Metabolism In Synechocystis Sp.PCC 6803

Cyanobacteria have evolved a CO2-concentrating mechanism(CCM)that functions to improve the efficiency of carbon fixation to deal with the varying carbon availability.Dissolved inorganic carbon(Ci=CO2+HCO3-)is transferred into cell through different specific transporters and fixed in a bacterial microcompartment that consists of a polyhedral protein shell filled with the CO2 fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase(RuBisCO).The ccm genes encoding different CCM components were regulated by Ci availability to optimize the cyanobacterial metabolic activity and growth velocity.In Synechocystis sp.PCC 6803,two LysR-type regulators,CmpR and NdhR,have been characterized as the key ccm regulators targeting the transcription of Ci transporters.The ntcB gene of Synechocystis sp.PCC 6803 was characterized as a LysR-type regulator involving in nitrate assimilation.In this study,we characterized the novel function of ntcB in the regulation of cyanobacterial CCM.The main results from this study are as follows:(1)Ci starvation remarkably induced ntcB up-regulation;(2)The deletion of ntcB inhibited the expressional level of ccmK1-ccmK2 and rbcL genes and delayed their up-regulation during the transition to Ci limitation process.Average number of carboxysomes per cell also changed when ntcB was inactivated;(3)Further bacterial one hybrid(B1H)assay demonstrated that NtcB did not regulate ccm genes except a LysR-type regulator encoding gene,sll1594.EMSA and B1H assay indicated a specific binding of NdhR to the promoter region of ntcB.This regulatory interaction of sil1594 and ntcB in response to Ci limitation also has been determined using RT-qPCR assay in mutant strains;(4)NtcA specificaly bind to the promoter region of ntcB,and the interaction of ntcA and ntcB was confirmed using B1H assay;(5)A transient up-regulation of nirA and ntcA during Ci limitation is dependent on the expression of both ndhR and These results exhibited a complex regulatory interaction among ntcA,ntcB and in cyanobacterial Ci limitation proeess.And NtcB may play an key role in the regulation of cellular C/N metabolism during Ci starvation...