title: Rhizobium meliloti fixGHI sequence predicts involvement of a specific cation pump in symbiotic nitrogen fixation.
abstract: We present genetic and structural analyses of a fix operon conserved among rhizobia, fixGHI from Rhizobium meliloti. The nucleotide sequence of the operon suggests it may contain a fourth gene, fixS. Adjacent open reading frames of this operon showed an overlap between TGA stop codons and ATG start codons in the form of an ATGA motif suggestive of translational coupling. All four predicted gene products contained probable transmembrane sequences. FixG contained two cysteine clusters typical of iron-sulfur centers and is predicted to be involved in a redox process. FixI was found to be homologous with P-type ATPases, particularly with K+ pumps from Escherichia coli and Streptococcus faecalis but also with eucaryotic Ca2+, Na+/K+, H+/K+, and H+ pumps, which implies that FixI is a pump of a specific cation involved in symbiotic nitrogen fixation. Since prototrophic growth of fixI mutants appeared to be unimpaired, the predicted FixI cation pump probably has a specifically symbiotic function. We suggest that the four proteins FixG, FixH, FixI, and FixS may participate in a membrane-bound complex coupling the FixI cation pump with a redox process catalyzed by FixG.
authors.affiliation: Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, Institut National de la Recherche Agronomique, Centre National de la Recherche Scientifique, Castanet-Tolosan, France.
authors.forename: D
authors.initials: D
authors.lastname: Kahn
authors.forename: M
authors.initials: M
authors.lastname: David
authors.forename: O
authors.initials: O
authors.lastname: Domergue
authors.forename: M L
authors.initials: ML
authors.lastname: Daveran
authors.forename: J
authors.initials: J
authors.lastname: Ghai
authors.forename: P R
authors.initials: PR
authors.lastname: Hirsch
authors.forename: J
authors.initials: J
authors.lastname: Batut
mesh_terms: D000251:Adenosine Triphosphatases / Q000235:genetics*; D000595:Amino Acid Sequence; D001483:Base Sequence; D004251:DNA Transposable Elements; D005796:Genes; D005798:Genes, Bacterial; D008969:Molecular Sequence Data; D005810:Multigene Family; D009586:Nitrogen Fixation; D010957:Plasmids; D015183:Restriction Mapping; D012231:Rhizobium / Q000235:genetics*; D013559:Symbiosis; D014161:Transduction, Genetic
chemical_list: D004251:DNA Transposable Elements; D000251:Adenosine Triphosphatases