Characterization of the Rhizobium (Sinorhizobium) meliloti high- and low-affinity phosphate uptake systems.
Genetic studies have suggested that Rhizobium (Sinorhizobium) meliloti contains two distinct phosphate (Pi) transport systems, encoded by the phoCDET genes and the orfA-pit genes, respectively. Here we present data which show that the ABC-type PhoCDET system has a high affinity for Pi (Km, 0.2 microM) and that Pi uptake by this system is severely inhibited by phosphonates. This high-affinity uptake system was induced under Pi-limiting conditions and was repressed in the presence of excess Pi. Uptake via the OrfA-Pit system was examined in (i) a phoC mutant which showed increased expression of the orfA-pit genes as a result of a promoter-up mutation and (ii) a phoB mutant (PhoB is required for phoCDET expression). Pi uptake in both strains exhibited saturation kinetics (Km, 1 to 2 microM) and was not inhibited by phosphonates. This uptake system was active in wild-type cells grown with excess Pi and appeared to be repressed when the cells were starved for Pi. Thus, our biochemical data show that the OrfA-Pit and PhoCDET uptake systems are differentially expressed depending on the state of the cell with respect to phosphate availability.
Voegele
R T
RT
Department of Biology, McMaster University, Hamilton, Ontario, Canada.
Bardin
S
S
Finan
T M
TM
D018528:ATP-Binding Cassette Transporters / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics; D001692:Biological Transport / Q000187:drug effects; D005798:Genes, Bacterial; D009154:Mutation; D009943:Organophosphorus Compounds / Q000494:pharmacology; D010710:Phosphates / Q000378:metabolism*; D016962:Sinorhizobium meliloti / Q000378:metabolism*
D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D009943:Organophosphorus Compounds; D010710:Phosphates; C061278:PhoB protein, Bacteria