title: The ntrBC genes of Azospirillum brasilense are part of a nifR3-like-ntrB-ntrC operon and are negatively regulated.
abstract: A cosmid able to complement the Nif- and nitrate-dependent growth phenotypes of the Azospirillum brasilense mutant FP9 was isolated from a genomic library of the wild-type strain FP2. A 6-kb DNA region was sequenced and showed two open reading frames (ORFs) identified as the ntrB and ntrC genes. An ORF1 located upstream from the ntrB gene and coding for a 36-kDa polypeptide showed similarity to the nifR3 gene of Rhodobacter capsulatus and the ORF1 of Rhizobium leguminosarum, both located upstream from the ntrB gene in a complex operon. Two other unidentified ORFs (ORF5 and partial ORF4) coding for hydrophobic polypeptides were also observed. delta ORF1-ntrBC, ORF1, ntrB, and ntrC mutants obtained by recombination of suicide plasmids containing an insertion of a promoterless lacZ kanamycin cassette showed decreased nitrogenase activities and were unable to grow on nitrate as the sole N source. These phenotypes were restored by complementation with plasmids containing the ntrC gene. Analysis of lacZ transcriptional fusions suggested that the ORF1-ntrBC operon in Azospirillum brasilense is expressed from a promoter located upstream from the ORF1 and that it is negatively regulated by the ntrC gene product.
authors.affiliation: Department of Pharmacology, Universidade Federal do Paraná, Curitiba, PR, Brazil.
authors.forename: H B
authors.initials: HB
authors.lastname: Machado
authors.forename: M G
authors.initials: MG
authors.lastname: Yates
authors.forename: S
authors.initials: S
authors.lastname: Funayama
authors.forename: L U
authors.initials: LU
authors.lastname: Rigo
authors.forename: M B
authors.initials: MB
authors.lastname: Steffens
authors.forename: E M
authors.initials: EM
authors.lastname: Souza
authors.forename: F O
authors.initials: FO
authors.lastname: Pedrosa
mesh_terms: D000595:Amino Acid Sequence; D015806:Azospirillum brasilense / Q000235:genetics*; D001483:Base Sequence; D002874:Chromosome Mapping; D003001:Cloning, Molecular; D003360:Cosmids; D004269:DNA, Bacterial / Q000235:genetics; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D007763:Lac Operon; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009586:Nitrogen Fixation / Q000235:genetics; D016366:Open Reading Frames; D009876:Operon; D011401:Promoter Regions, Genetic; D017386:Sequence Homology, Amino Acid
chemical_list: D004269:DNA, Bacterial