title: Structural and physiological analyses of the alkanesulphonate-binding protein (SsuA) of the citrus pathogen Xanthomonas citri.
abstract: BACKGROUND
  The uptake of sulphur-containing compounds plays a pivotal role in the physiology of bacteria that live in aerobic soils where organosulfur compounds such as sulphonates and sulphate esters represent more than 95% of the available sulphur. Until now, no information has been available on the uptake of sulphonates by bacterial plant pathogens, particularly those of the Xanthomonas genus, which encompasses several pathogenic species. In the present study, we characterised the alkanesulphonate uptake system (Ssu) of Xanthomonas axonopodis pv. citri 306 strain (X. citri), the etiological agent of citrus canker.
  
  
  RESULTS
  A single operon-like gene cluster (ssuEDACB) that encodes both the sulphur uptake system and enzymes involved in desulphurisation was detected in the genomes of X. citri and of the closely related species. We characterised X. citri SsuA protein, a periplasmic alkanesulphonate-binding protein that, together with SsuC and SsuB, defines the alkanesulphonate uptake system. The crystal structure of SsuA bound to MOPS, MES and HEPES, which is herein described for the first time, provides evidence for the importance of a conserved dipole in sulphate group coordination, identifies specific amino acids interacting with the sulphate group and shows the presence of a rather large binding pocket that explains the rather wide range of molecules recognised by the protein. Isolation of an isogenic ssuA-knockout derivative of the X. citri 306 strain showed that disruption of alkanesulphonate uptake affects both xanthan gum production and generation of canker lesions in sweet orange leaves.
  
  
  CONCLUSIONS
  The present study unravels unique structural and functional features of the X. citri SsuA protein and provides the first experimental evidence that an ABC uptake system affects the virulence of this phytopathogen.
authors.affiliation: Departamento de Microbiologia, Universidade de São Paulo, São Paulo, São Paulo, Brazil.
authors.forename: Fabiano
authors.initials: F
authors.lastname: Tófoli de Araújo
authors.forename: Victor M
authors.initials: VM
authors.lastname: Bolanos-Garcia
authors.forename: Cristiane T
authors.initials: CT
authors.lastname: Pereira
authors.forename: Mario
authors.initials: M
authors.lastname: Sanches
authors.forename: Elisa E
authors.initials: EE
authors.lastname: Oshiro
authors.forename: Rita C C
authors.initials: RC
authors.lastname: Ferreira
authors.forename: Dimitri Y
authors.initials: DY
authors.lastname: Chigardze
authors.forename: João Alexandre Gonçalves
authors.initials: JA
authors.lastname: Barbosa
authors.forename: Luís Carlos
authors.initials: LC
authors.lastname: de Souza Ferreira
authors.forename: Celso E
authors.initials: CE
authors.lastname: Benedetti
authors.forename: Tom L
authors.initials: TL
authors.lastname: Blundell
authors.forename: Andrea
authors.initials: A
authors.lastname: Balan
mesh_terms: D000476:Alkanesulfonates / Q000737:chemistry* / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D001665:Binding Sites; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D032084:Citrus sinensis / Q000254:growth & development / Q000378:metabolism / Q000382:microbiology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010935:Plant Diseases / Q000382:microbiology; D011135:Polysaccharides, Bacterial / Q000378:metabolism; D016415:Sequence Alignment; D014774:Virulence; D014961:X-Ray Diffraction; D014974:Xanthomonas / Q000235:genetics / Q000378:metabolism / Q000472:pathogenicity*
chemical_list: D000476:Alkanesulfonates; D001426:Bacterial Proteins; D002352:Carrier Proteins; D011135:Polysaccharides, Bacterial; C002563:xanthan gum