title: Expanding the regulatory network that controls nitrogen fixation in Sinorhizobium meliloti: elucidating the role of the two-component system hFixL-FxkR.
abstract: In Sinorhizobium meliloti, nitrogen fixation is regulated in response to oxygen concentration through the FixL-FixJ two-component system (TCS). Besides this conserved TCS, the field isolate SM11 also encodes the hFixL-FxkR TCS, which is responsible for the microoxic response in Rhizobium etli. Through genetic and physiological assays, we evaluated the role of the hFixL-FxkR TCS in S. meliloti SM11. Our results revealed that this regulatory system activates the expression of a fixKf orthologue (fixKa), in response to low oxygen concentration. Null mutations in either hFixL or FxkR promote upregulation of fixK1, a direct target of FixJ. Furthermore, the absence of this TCS translates into higher nitrogen fixation values as well as higher expression of fixN1 in nodules. Individual mutations in each of the fixK-like regulators encoded in the S. meliloti SM11 genome do not completely restrict fixN1 or fixN2 expression, pointing towards redundancy among these regulators. Both copies of fixN are necessary to achieve optimal levels of nitrogen fixation. This work provides evidence that the hFixL-FxkR TCS is activated in response to low oxygen concentration in S. meliloti SM11 and that it negatively regulates the expression of fixK1, fixN1 and nitrogen fixation.
authors.affiliation: Programa de Dinámica Genómica, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, Mexico.|Instituto de Investigaciones Básicas y Aplicadas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos, Mexico.
authors.forename: Alma
authors.initials: A
authors.lastname: Reyes-González
authors.affiliation: Programa de Dinámica Genómica, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, Mexico.
authors.forename: Chouhra
authors.initials: C
authors.lastname: Talbi
authors.affiliation: Programa de Dinámica Genómica, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, Mexico.
authors.forename: Susana
authors.initials: S
authors.lastname: Rodríguez
authors.affiliation: Programa de Dinámica Genómica, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, Mexico.
authors.forename: Patricia
authors.initials: P
authors.lastname: Rivera
authors.affiliation: Department of Microbiology and Environmental Toxicology, University of California, Santa Cruz, USA.
authors.forename: David
authors.initials: D
authors.lastname: Zamorano-Sánchez
authors.affiliation: Programa de Dinámica Genómica, Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, Mexico.
authors.forename: Lourdes
authors.initials: L
authors.lastname: Girard
mesh_terms: D000693:Anaerobiosis / Q000502:physiology; D001426:Bacterial Proteins / Q000096:biosynthesis* / Q000235:genetics* / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial / Q000235:genetics; D006420:Hemeproteins / Q000235:genetics* / Q000378:metabolism; D000071677:Histidine Kinase; D007874:Leghemoglobin / Q000378:metabolism; D000455:Medicago sativa / Q000382:microbiology*; D008565:Membrane Proteins / Q000096:biosynthesis* / Q000378:metabolism; D009586:Nitrogen Fixation / Q000235:genetics*; D010100:Oxygen / Q000378:metabolism; D010957:Plasmids / Q000235:genetics; D053204:Root Nodules, Plant / Q000378:metabolism* / Q000382:microbiology; D016962:Sinorhizobium meliloti / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; C087009:FixN protein, bacteria; D006420:Hemeproteins; D007874:Leghemoglobin; D008565:Membrane Proteins; C066628:FixK protein, Bacteria; C067929:FixL protein, Bacteria; D000071677:Histidine Kinase; D010100:Oxygen