title: Heterologous Complementation Reveals a Specialized Activity for BacA in the Medicago-Sinorhizobium meliloti Symbiosis.
abstract: The bacterium Sinorhizobium meliloti Rm2011 forms N2-fixing root nodules on alfalfa and other leguminous plants. The pSymB chromid contains a 110-kb region (the ETR region) showing high synteny to a chromosomally located region in Sinorhizobium fredii NGR234 and related rhizobia. We recently introduced the ETR region from S. fredii NGR234 into the S. meliloti chromosome. Here, we report that, unexpectedly, the S. fredii NGR234 ETR region did not complement deletion of the S. meliloti ETR region in symbiosis with Medicago sativa. This phenotype was due to the bacA gene of NGR234 not being functionally interchangeable with the S. meliloti bacA gene during M. sativa symbiosis. Further analysis revealed that, whereas bacA genes from S. fredii or Rhizobium leguminosarum bv. viciae 3841 failed to complement the Fix-phenotype of a S. meliloti bacA mutant with M. sativa, they allowed for further developmental progression prior to a loss of viability. In contrast, with Melilotus alba, bacA from S. fredii and R. leguminosarum supported N2fixation by a S. meliloti bacA mutant. Additionally, the S. meliloti bacA gene can support N2fixation of a R. leguminosarum bacA mutant during symbiosis with Pisum sativum. A phylogeny of BacA proteins illustrated that S. meliloti BacA has rapidly diverged from most rhizobia and has converged toward the sequence of pathogenic genera Brucella and Escherichia. These data suggest that the S. meliloti BacA has evolved toward a specific interaction with Medicago and highlights the limitations of using a single model system for the study of complex biological topics.
authors.affiliation: Department of Biology, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S 4K1, Canada.
authors.forename: George C
authors.initials: GC
authors.lastname: diCenzo
authors.affiliation: Department of Biology, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Maryam
authors.initials: M
authors.lastname: Zamani
authors.affiliation: Department of Biology, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Hannah N
authors.initials: HN
authors.lastname: Ludwig
authors.affiliation: Department of Biology, McMaster University, 1280 Main St. W., Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Turlough M
authors.initials: TM
authors.lastname: Finan
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D015870:Gene Expression; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D056426:Genetic Loci; D029902:Medicago / Q000382:microbiology*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D009586:Nitrogen Fixation / Q000235:genetics; D018532:Pisum sativum / Q000382:microbiology; D010641:Phenotype; D010802:Phylogeny; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000378:metabolism*; D013559:Symbiosis
chemical_list: D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; C082397:bacA protein, Rhizobium meliloti