Functional Genomics Research On The Pleiotropic Transcriptional Regulator MucR1 In Sinorhizobium Fredii CCBAU45436

Rhizobia need key symbiosis genes(nod/nif)geared for nodulation and nitrogen fixation to establish mutually beneficial symbiosis with legumes.Based on this mutualism,people inoculate rhizobia when growing legume crops and/or forages to improve the yield and quality,and meanwhile,to reduce the use of industrial nitrogen fertilizer.In today's agricultural production,screening for strains possessing high nitrogen efficiency and strong competitiveness is still the main corncern in rhizobial application.It's well-known that the efficiency of symbiotic nitrogen fixation mainly depends on the rhizobial coordination of their metabolic networks to adapt to the endocellular environment.In this study,we focused on how MucR1,a pleiotropic zinc finger bearing transcriptional regulator in Sinorhizobium fredii CCBAU45436,affects symbiotic nitrogen fixation between this strain and its soybean host(Glycine max cv.JD17).MucR1 belongs to MucR/RosR family which is widely distributed in the a-proteobacteria.Previous work in our lab have shown that deletion of mucRl in S.fredii CCBAU45436 led to a reduced production of exopolysaccharide(EPS').And in this study,we found that mucRl was also critical for efficient symbiotic nitrogen fixation��mucRl mutant formed smaller and Fix-nodules on G.max cv.JD17 roots.Nodulation test on strains derived from mucRl mutant that expressing varous mucR homologues and their derivants revealed that symbiotic defects induced mucRl mutation was depended on a frame-shift mutation in mucR2.To uncover the regulator networks of MucR1 in S.fredii CCBAU45436,combined RNA-seq and ChIP-seq analysis was performed.The high-throughput sequencing,together with biochemical and genetic eviences revealed that:(?)MucR1 is required for active expression of ABC-transporters(Fe,Mo,S,P,Zn)but not for nif/fix genes to maintain the symbiotic nitrogen fixation in nodules;(?)MucRl is a pleotropic multi-level regulator and its regulon exhibites notable codition-dependence;(?)MucR1 binds to AT-rich DNA with bias and the majority of MucR1 target genes are foreign genes obtained by horizontal gene transfer;(?)MucR1 target genes are mainly subject to its negative regulation;(?)MucR1 target genes are enriched in COG clusters involved in transcription and signal transduction,overexpression of 5 LuxR family regulators in S.fredii CCBAU45436 lead to Nod-.This study shed light on how MucR1 functions in symbiotic nitrogen fixation regulation,and more importantly redefined this ancient zinc finger protein as a foreign gene silencer.The new understandings about MucR1 will benefit the further studies on the regulatory networks behind legume-rhizobium symbiosis and the evolution of rhizobial genomes.