title: The quorum sensing regulator CinR hierarchically regulates two other quorum sensing pathways in ligand-dependent and -independent fashions in Rhizobium etli.
abstract: Many rhizobial species use complex N-acyl-homoserine lactone (AHL)-based quorum sensing (QS) systems to monitor their population density and regulate their symbiotic interactions with their plant hosts. There are at least three LuxRI-type regulatory systems in Rhizobium etli CFN42: CinRI, RaiRI, and TraRI. In this study, we show that CinI, RaiI, and TraI are responsible for synthesizing all AHLs under the tested conditions. The activation of these AHL synthase genes requires their corresponding LuxR-type counterparts. We further demonstrate that CinRI is at the top of the regulatory cascade that activates RaiRI and TraRI QS systems. Moreover, we discovered that CinR possesses a specific affinity to bind cinI promoter in the absence of its cognate AHL ligand, thereby activating cinI transcription. Addition of AHLs leads to improved binding to the cinI promoter and enhanced cinI expression. Furthermore, we found that compared to the wild type, the cinR mutation displayed reduced nodule formation, and cinR, raiR, and traI mutants show significantly lower levels of nitrogen fixation activity than the wild type. These results suggest that the complex QS regulatory systems in R. etli play an important role in its symbiosis with legume hosts.
  
  
  OBJECTIVE
  Many bacteria use quorum sensing (QS) to monitor their cell densities and coordinately regulate a number of physiological functions. Rhizobia often have diverse and complex LuxR/LuxI-type quorum sensing systems that may be involved in symbiosis and N2 fixation. In this study, we identified three LuxR/LuxI-type QS systems in Rhizobium etli CFN42: CinRI, RaiRI, and TraRI. We established a complex network of regulation between these QS components and found that these QS systems played important roles in symbiosis processes.
authors.affiliation: Department of Microbiology, Nanjing Agricultural University, Nanjing, China.
authors.forename: Huiming
authors.initials: H
authors.lastname: Zheng
authors.affiliation: Department of Microbiology, Nanjing Agricultural University, Nanjing, China.
authors.forename: Yiling
authors.initials: Y
authors.lastname: Mao
authors.affiliation: Department of Microbiology, Nanjing Agricultural University, Nanjing, China.
authors.forename: Qingcheng
authors.initials: Q
authors.lastname: Zhu
authors.affiliation: Department of Microbiology, Nanjing Agricultural University, Nanjing, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Ling
authors.affiliation: Department of Microbiology, Nanjing Agricultural University, Nanjing, China.
authors.forename: Na
authors.initials: N
authors.lastname: Zhang
authors.affiliation: Department of Microbiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
authors.forename: Nawar
authors.initials: N
authors.lastname: Naseer
authors.affiliation: Department of Microbiology, Nanjing Agricultural University, Nanjing, China mol_microb@njau.edu.cn junzhu@mail.med.upenn.edu.
authors.forename: Zengtao
authors.initials: Z
authors.lastname: Zhong
authors.affiliation: Department of Microbiology, Nanjing Agricultural University, Nanjing, China Department of Microbiology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA mol_microb@njau.edu.cn junzhu@mail.med.upenn.edu.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhu
mesh_terms: D054742:Acyl-Butyrolactones / Q000378:metabolism*; D004269:DNA, Bacterial / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D011485:Protein Binding; D053038:Quorum Sensing; D042163:Rhizobium etli / Q000235:genetics* / Q000502:physiology*; D014157:Transcription Factors / Q000378:metabolism*
chemical_list: D054742:Acyl-Butyrolactones; D004269:DNA, Bacterial; D014157:Transcription Factors