Phosphate Uptake in the Cyanobacterium Synechococcus R-2 PCC 7942

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Phosphate uptake rates in Synechococcus R-2 in BG-11 media (a nitrate-based medium, not phosphate limited) were measured using cells grown semi-continuously and in continuous culture. Net uptake of phosphate is proportional to external concentration. Growing cells at pH. ~O have a net uptake rate of about 600 μmol m^<-2> s^<-1> phosphate, but the isotopic flux for <32>^P phosphate was about 4 nmol m^<-2> s^<-1>. There appears to be a constitutive over-capacity for phosphate uptake. The K_m and V_<max> of the saturable component were not significantly different at pH_0 7.5 and 10, hence the transport system probably recognizes both H_2PO^-_4 and HP0^<2->_4.The intracellular inorganic phosphate concentration is about 3 to 10 mol m^lt;-3>, but there is an intracellular polyphosphate store of about 400 mol m^<-3>. Intracellular inoiganic phosphate is 25 to 50 kJ molm<-1> from electrochemical equilibrium in both the light and dark and at pH. 7.5 and 10. Phosphate uptake is very slow in the dark (≈ 100 pmol m^<-2> s^<-1>) and is light-activated (pH_0 7.5 ≈ 1.3 nmol m^<-2>s^<-1>, pH_0 10 ≈ 600 pmol m^<-2>s^<-1>). Up-take has an irreversible requirement for Mg^<2+> in the medium. Uptake in the light is strongly Na^+_dependent. Phosphate uptake was negatively electrogenic (net negative charge taken up when transporting phosphate) at pH* 7.5, but positively electrogenic at pH_0. This seems to exclude a sodium motive force driven mechanism. An ATP-driven phosphate uptake mechanism needs to have a stoichiometry of one phosphate taken up per ATP (1 P0_<4 in>/ATP) to be thermodynamically possible under all the conditions tested in the present study.
日本植物生理学会の論文

Phosphate Uptake in the Cyanobacterium Synechococcus R-2 PCC 7942

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