PHOSPHATE-UPTAKE IN THE CYANOBACTERIUM SYNECHOCOCCUS R-2 PCC-7942

Phosphate uptake rates in Synechococcus R-2 in BG-11 media (a nitrate- based medium, not phosphate limited) were measured using cells grown s emi-continuously and in continuous culture. Net uptake of phosphate is proportional to external concentration. Growing cells at pH(o) 10 hav e a net uptake rate of about 600 pmol m(-2) s(-1) phosphate, but the i sotopic flux for P-32 phosphate was about 4 nmol m(-2) s(-1). There ap pears 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(o) 7.5 and 10, hence the transport system probably recognizes b oth H2PO4- and HPO42-. The intracellular inorganic phosphate concentra tion is about 3 to 10 mol m(-3), but there is an intracellular polypho sphate store of about 400 mol m(-3) Intracellular inorganic phosphate is 25 to 50 kJ mol(-1) from electrochemical equilibrium in both the li ght and dark and at pH, 7.5 and 10. Phosphate uptake is very slow in t he dark (approximate to 100 pmol m(-2) s(-1)) and is light-activated ( pH(o) 7.5 approximate to 1.3 nmol m(-2) s(-1), pH(o) 10 approximate to 600 pmol m(-2) s(-1)). Uptake has an irreversible requirement for Mg2 + in the medium. Uptake in the light is strongly Na+-dependent. Phosph ate uptake was negatively electrogenic (net negative charge taken up w hen transporting phosphate) at pH(o) 7.5, but positively electrogenic at pH(o) 10. This seems to exclude a sodium motive force driven mechan ism. An ATP-driven phosphate uptake mechanism needs to have a stoichio metry of one phosphate taken up per ATP (1 PO4 in/ATP) to be thermodyn amically possible under all the conditions tested in the present study .