title: The Paracoccus denitrificans NarK-like nitrate and nitrite transporters-probing nitrate uptake and nitrate/nitrite exchange mechanisms.
abstract: Nitrate and nitrite transport across biological membranes is often facilitated by protein transporters that are members of the major facilitator superfamily. Paracoccus denitrificans contains an unusual arrangement whereby two of these transporters, NarK1 and NarK2, are fused into a single protein, NarK, which delivers nitrate to the respiratory nitrate reductase and transfers the product, nitrite, to the periplasm. Our complementation studies, using a mutant lacking the nitrate/proton symporter NasA from the assimilatory nitrate reductase pathway, support that NarK1 functions as a nitrate/proton symporter while NarK2 is a nitrate/nitrite antiporter. Through the same experimental system, we find that Escherichia coli NarK and NarU can complement deletions in both narK and nasA in P. denitrificans, suggesting that, while these proteins are most likely nitrate/nitrite antiporters, they can also act in the net uptake of nitrate. Finally, we argue that primary sequence analysis and structural modelling do not readily explain why NasA, NarK1 and NarK2, as well as other transporters from this protein family, have such different functions, ranging from net nitrate uptake to nitrate/nitrite exchange.
authors.affiliation: School of Life Sciences, University of Lincoln, Lincoln, LN6 7TS, UK.|School of Life and Health Sciences, Aston University, Birmingham, B4 7ET, UK.|Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, UK.
authors.forename: Alan D
authors.initials: AD
authors.lastname: Goddard
authors.affiliation: Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, UK.
authors.forename: Shilpa
authors.initials: S
authors.lastname: Bali
authors.affiliation: Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, UK.|MRC Centre for Molecular Bacteriology and Infection, Department of Life Sciences, Imperial College London, Kensington, London, SW7 2DD, UK.
authors.forename: Despoina A I
authors.initials: DA
authors.lastname: Mavridou
authors.affiliation: Centre for Molecular and Structural Biochemistry, School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.|Departamento de Bioquímica y Biología Molecular, Universidad de Córdoba, Edificio Severo Ochoa, 1a planta, Campus de Rabanales, Córdoba, 14071, Spain.
authors.forename: Victor M
authors.initials: VM
authors.lastname: Luque-Almagro
authors.affiliation: Centre for Molecular and Structural Biochemistry, School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Gates
authors.affiliation: Departamento de Bioquímica y Biología Molecular, Universidad de Córdoba, Edificio Severo Ochoa, 1a planta, Campus de Rabanales, Córdoba, 14071, Spain.
authors.forename: M
authors.initials: M
authors.lastname: Dolores Roldán
authors.affiliation: Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, UK.
authors.forename: Simon
authors.initials: S
authors.lastname: Newstead
authors.affiliation: Centre for Molecular and Structural Biochemistry, School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.
authors.forename: David J
authors.initials: DJ
authors.lastname: Richardson
authors.affiliation: Department of Biochemistry, University of Oxford, South Parks Road, Oxford, OX1 3QU, UK.
authors.forename: Stuart J
authors.initials: SJ
authors.lastname: Ferguson
mesh_terms: D027321:Anion Transport Proteins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism; D001692:Biological Transport; D004926:Escherichia coli / Q000378:metabolism; D005816:Genetic Complementation Test; D017136:Ion Transport; D050901:Nitrate Reductase / Q000378:metabolism; D000091285:Nitrate Transporters; D009566:Nitrates / Q000378:metabolism; D009572:Nitrite Reductases / Q000378:metabolism; D009573:Nitrites / Q000378:metabolism; D010231:Paracoccus denitrificans / Q000378:metabolism*
chemical_list: D027321:Anion Transport Proteins; D001426:Bacterial Proteins; D000091285:Nitrate Transporters; D009566:Nitrates; D009573:Nitrites; D009572:Nitrite Reductases; D050901:Nitrate Reductase