Abstract
Background: Most 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, Delta Pst1 and Delta Pst2, 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.
Results: Growth rates of the two mutants and wild type were similar either under phosphate-sufficient or deficient condition. The K-m for phosphate uptake was 6.09 mu M in wild type and this was reduced to 0.13 mu in Delta Pst1 cells and 5.16 mu M in the Delta Pst2 strain. The V-max values of 2.48, 0.22, and 2.17 mu mol . (min . mg of chlorophyll a)(-1) were obtained for wild type, the Delta Pst1 and Delta Pst2 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.
Conclusion: The data demonstrate that Pst2 system of Synechocystis has higher affinity toward phosphate with lower V-max than Pst1 system. The Pst1 system had similar K-m and V-max values to those of the wild type suggesting that Pst1 is the main phosphate transporter in Synechocystis sp. PCC 6803. The K-m 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.