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Abstract
BackgroundMost living cells uptake phosphate, an indispensable nutrient for growth from their natural environment. In Synechocystis sp. PCC 6803, the cells lack phosphate-inorganic transport (Pit) system but contain two phosphate-specific transport (Pst) systems, Pst1 and Pst2. We investigated the kinetics of Pi uptake of these two Pst systems by constructing the two mutants, ΔPst1 and ΔPst2, and comparing their kinetic properties with those of the wild-type cells under both Pi-sufficient and deficient conditions. The effects of pH and Na+ on the uptake of phosphate in Synechocystis were also studied.ResultsGrowth rates of the two mutants and wild type were similar either under phosphate-sufficient or deficient condition. The Km for phosphate uptake was 6.09 μM in wild type and this was reduced to 0.13 μM in ΔPst1 cells and 5.16 μM in the ΔPst2 strain. The Vmax values of 2.48, 0.22, and 2.17 μmol • (min • mg of chlorophyll a)-1 were obtained for wild type, the ΔPst1 and ΔPst2 strains, respectively. A monophasic phosphate uptake was observed in wild-type cells. The uptake of phosphate was energy and pH-dependent with a broad pH optimum between pH 7-10. Osmolality imposed by NaCl stimulated phosphate uptake whereas that imposed by sorbitol decreased uptake, suggesting stimulation of uptake was dependent upon ionic effects.ConclusionThe data demonstrate that Pst2 system of Synechocystis has higher affinity toward phosphate with lower Vmax than Pst1 system. The Pst1 system had similar Km and Vmax values to those of the wild type suggesting that Pst1 is the main phosphate transporter in Synechocystis sp. PCC 6803. The Km of Pst1 of Synechocystis is closer to that of Pit system than to that of the Pst system of E. coli, suggesting that Synechocystis Pst1 is rather a medium/low affinity transporter whereas Pst2 is a high affinity transporter.
Highlights
Most living cells uptake phosphate, an indispensable nutrient for growth from their natural environment
Mg of chlorophyll a)-1 were obtained for wild type, the ΔPst1 and ΔPst2 strains, respectively
Osmolality imposed by NaCl stimulated phosphate uptake whereas that imposed by sorbitol decreased uptake, suggesting stimulation of uptake was dependent upon ionic effects
Summary
Most living cells uptake phosphate, an indispensable nutrient for growth from their natural environment. Inorganic phosphate (Pi), the only form of phosphorus that can be directly utilized by cells, is often limiting in natural environments where it is frequently present at nanomolar levels [1]. PCC 7942 exhibits a lower Km for Pi uptake when grown under Pi-limiting conditions. This organism contains both low-affinity and high-affinity Pi transport systems where the high-affinity Pi transport activity is regulated by the periplasmic Pibinding protein SphX [8]. PCC 6803 (hereafter Synechocystis 6803) [9] This cyanobacterium was previously shown to contain two Pst systems, Pst and Pst, that are up-regulated in response to Pi limitation [4]. We demonstrated that Pst was the main Pi transporter whereas Pst might play a role in the uptake of Pi under low Pi environments
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