Vanadium Requirements and Uptake Kinetics in the Dinitrogen-Fixing Bacterium Azotobacter vinelandii

Abstract

Vanadium is a cofactor in the alternative V-nitrogenase that is expressed by some N 2 -fixing bacteria when Mo is not available. We investigated the V requirements, the kinetics of V uptake, and the production of catechol compounds across a range of concentrations of vanadium in diazotrophic cultures of the soil bacterium Azotobacter vinelandii. In strain CA11.70, a mutant that expresses only the V-nitrogenase, V concentrations in the medium between 10 ؊8 and 10 ؊6 M sustain maximum growth rates; they are limiting below this range and toxic above. A. vinelandii excretes in its growth medium micromolar concentrations of the catechol siderophores azotochelin and protochelin, which bind the vanadate oxoanion. The production of catechols increases when V concentrations become toxic. Short-term uptake experiments with the radioactive isotope 49 V show that bacteria take up the V-catechol complexes through a regulated transport system(s), which shuts down at high V concentrations. The modulation of the excretion of catechols and of the uptake of the V-catechol complexes allows A. vinelandii to precisely manage its V homeostasis over a range of V concentrations, from limiting to toxic.

Magnetic susceptibility of Mn(III) complexes of hydroxamate siderophores

“…While ligand-mediated uptake of metals other than iron has been demonstrated recently for molybdenum and vanadium with siderophores [3,34] and for copper with chalkophores [35,36], ligand-mediated uptake of manganese has not been reported to date. Nevertheless, siderophores may play an important role in biogenic cycling of manganese [10,19,37,38] .…”

Metallophores and Trace Metal Biogeochemistry

“…Azotochelin, protochelin (Bellenger et al 2008b), and azotobactin (Wichard et al 2009a) can promote the uptake of molybdate for use in nitrogenase (Bellenger et al 2011;Holler et al 2009) when it is present at low concentrations (\10 -8 M), even in iron replete media. In addition, at high molybdate concentrations ([10 -6 -10 -5 M), the uptake of Mo is stopped, possibly to avoid the toxic effects of the metal (Bellenger et al 2008a;Cornish and Page 2000). Under Modepleted conditions, these metallophores promote the uptake of V at low concentrations, but uptake is halted in Mo-replete media (Bellenger et al 2008a, b).…”

“…Redox enzymes, most notably Fe-Mo nitrogenase (Howard and Rees 2006) and other N-cycle enzymes (Self et al 2001) Azotochelin, protochelin, and aminochelin promote solubilization and uptake (Bellenger et al 2008b;Cornish and Page 1995); pyrochelin may affect transport in Pseudomonas aeruginosa (Visca et al 1992) V Fe-V nitrogenase (Robson et al 1986) Azotochelin and protochelin promote uptake (Bellenger et al 2008a) Ni Nine metalloenzymes (Mulrooney and Hausinger 2003), most notably urease (Fraústo da Silva and Williams 2001), nitrogenase (Fraústo da Silva and Williams 2001), and superoxide dismutase (Dupont et al 2008) No metallophores have been identified, but an uptake system is suspected in E. coli (Dosanjh and Michel 2006); pyrochelin may affect Ni(II) uptake in Pseudomonas aeruginosa (Visca et al 1992); interactions with known Fe(III)-siderophores are of moderated strength (Hernlem et al 1996) Co B 12 , substitution into urease or carbonic anhydrase; other unknown functions (Fraústo da Silva and Williams 2001; Morel 2008; Morel et al 2006) Uptake is accelerated by unknown biogenic ligands for cyanobacteria (Saito et al 2002(Saito et al , 2005; Co(III) is bound very strongly by Fe(III)-siderophores, and thus natural marine metallophores may be structurally similar (Duckworth et al 2009c); pyrochelin may affect Co(II) transport in P. aeruginosa (Visca et al 1992)…”

“…In oxic environments, the dominant iron species are likely to be mineral phases (viz., oxides and hydroxides), although organic complexes may also be present. Molybdate is likely to sorb to the surface of minerals as monomers or polymers (Arai 2010), or be bound to natural organic matter (Wichard et al 2009b); vanadium is likely to be found in similar pools in most environments (Adriano 2001;Bellenger et al 2008a). The surface sorbed pool of oxyanions is likely to be more labile than the Fe(III) associated with mineral phases, and rates of exchange of the oxyanions from organic ligands greatly exceed those of Fe(III) ).…”

Multiple roles of siderophores in free-living nitrogen-fixing bacteria

“…When Mo is not available but V is present in the growth medium, A. vinelandii expresses the V-nitrogenase. Growth curves of a mutant strain that expresses only the V-nitrogenase (CA11.70, Joerger et al 1989) show also two distinct growth phases when the bacteria are grown on low [V] (Bellenger et al 2008a). A concentration of [V] = 5 9 10 -8 M is required for maximum initial growth rates (Fig.…”