Inorganic phosphate transport inAcinetobacter lwoffi

Characterization of two inducible phosphate transport systems in Rhizobium tropici

Rhizobium tropici forms nitrogen-fixing nodules on the roots of the common bean (Phaseolus vulgaris). Like other legume-Rhizobium symbioses, the bean-R. tropici association is sensitive to the availability of phosphate (P(i)). To better understand phosphorus movement between the bacteroid and the host plant, P(i) transport was characterized in R. tropici. We observed two P(i) transport systems, a high-affinity system and a low-affinity system. To facilitate the study of these transport systems, a Tn5B22 transposon mutant lacking expression of the high-affinity transport system was isolated and used to characterize the low-affinity transport system in the absence of the high-affinity system. The K(m) and V(max) values for the low-affinity system were estimated to be 34 +/- 3 microM P(i) and 118 +/- 8 nmol of P(i) x min(-1) x mg (dry weight) of cells(-1), respectively, and the K(m) and V(max) values for the high-affinity system were 0.45 +/- 0.01 microM P(i) and 86 +/- 5 nmol of P(i) x min(-1) x mg (dry weight) of cells(-1), respectively. Both systems were inducible by P(i) starvation and were also shock sensitive, which indicated that there was a periplasmic binding-protein component. Neither transport system appeared to be sensitive to the proton motive force dissipator carbonyl cyanide m-chlorophenylhydrazone, but P(i) transport through both systems was eliminated by the ATPase inhibitor N,N'-dicyclohexylcarbodiimide; the P(i) transport rate was correlated with the intracellular ATP concentration. Also, P(i) movement through both systems appeared to be unidirectional, as no efflux or exchange was observed with either the wild-type strain or the mutant. These properties suggest that both P(i) transport systems are ABC type systems. Analysis of the transposon insertion site revealed that the interrupted gene exhibited a high level of homology with kdpE, which in several bacteria encodes a cytoplasmic response regulator that governs responses to low potassium contents and/or changes in medium osmolarity.

Biology of polyphosphate-accumulating bacteria involved in enhanced biological phosphorus removal

Recent research on the process of biological phosphorus removal in lab-scale treatment systems has indicated that: (i) the development of an actively polyP-accumulating bacterial community after the introduction of an anaerobic period may take at least 4 months; (ii) up to 80% of all aerobic bacteria isolated from these communities are able to accumulate polyP; (iii) polyP synthesized by the bacterial communities of lab-scale treatment systems is probably mainly low polymeric, not exceeding 20 P-residues, and this polyP is rapidly degraded during the anaerobic period; (iv) the enzymatic hydrolysis of polyP under anaerobic conditions is accompanied by PHB formation from exogenous acetate, reducing equivalents are provided by the degradation of carbohydrates; and (v) nitric oxide inhibits the release of phosphate under anaerobic conditions in Renpho and F&D sludges. Bacteria belonging to the genus Acinetobacter occur in a wide variety of activated sludges in which enhanced biological phosphate removal is observed. A. johnsonii 210A was studied in detail with respect to the elemental composition of polyP granules, the enzymatic synthesis and degradation of polyP, the regulation of polyP metabolism, and the transport of phosphate. A. johnsonii 210A reflects activated sludge in a number of ways as far as polyP metabolism is concerned but its polyP is highly polymeric and the phosphate efflux rate under anaerobic conditions is relatively low and not increased by exogenous acetate. In addition to Acinetobacter, other polyP-accumulating microorganisms may be involved in biological phosphorus removal. The isolation of polyP-accumulating denitrifying bacteria may well have interesting implications for a new process design in wastewater treatment systems. Further studies on the enzymes involved in polyP biosynthesis and on the uptake and efflux systems of phosphate, potassium, magnesium and lower fatty acids in pure cultures will enlarge our insight in the energetics of the metabolism of polyP. In addition, the regulation of the metabolism of polyP-accumulating organisms needs to be studied in more detail.

Translocation of metal phosphate via the phosphate inorganic transport system of Escherichia coli

Pi transport via the phosphate inorganic transport system (Pit) of Escherichia coli was studied in natural and artificial membranes. Pi uptake via Pit is dependent on the presence of divalent cations, like Mg2+, Ca2+, Co2+, or Mn2+, which form a soluble, neutral metal phosphate (MeHPO4) complex. Pi-dependent uptake of Mg2+ and Ca2+, equimolar cotransport of Pi and Ca2+, and inhibition by Mg2+ of Ca2+ uptake in the presence of Pi, but not of Pi uptake in the presence of Ca2+, indicate that a metal phosphate complex is the transported solute. Metal phosphate is transported in symport with H+ with a mechanistic stoichiometry of 1. Pit mediates efflux and homologous exchange of metal phosphate, but not heterologous metal phosphate exchange with Pi, glycerol-3P, or glucose-6P. The metal phosphate efflux rate increased with pH, whereas the rate of metal phosphate exchange was essentially pH independent. Metal phosphate uptake was inhibited at low internal pH. Efflux was inhibited by a proton motive force (interior negative and alkaline), whereas exchange was inhibited by the membrane potential only. These results have been evaluated in terms of ordered binding and dissociation of metal phosphate and proton on the outer and inner surface of the cytoplasmic membrane.