Characterization of spermidine transport system in a cyanobacterium,synechocystis sp. PCC 6803

The transport of spermidine into a cyanobacterium, Synechocystis sp. PCC 6803, was characterized by measuring the uptake of 14C-spermidine. Spermidine transport was shown to be saturable with an apparent affinity constant (Km) value of 67 microM and a maximal velocity (Vmax) value of 0.45 nmol/min/mg protein. Spermidine uptake was pHdependent with the pH optimum being 8.0. The competition experiment showed strong inhibition of spermidine uptake by putrescine and spermine, whereas amino acids were hardly inhibitory. The inhibition kinetics of spermidine transport by putrescine and spermine was found to be noncompetitive with Ki values of 292 and 432 microM, respectively. The inhibition of spermidine transport by various metabolic inhibitors and ionophores suggests that spermidine uptake is energy-dependent. The diminution of cell growth was observed in cells grown at a high concentration of NaCl. Addition of a low concentration of spermidine at 0.5 mM relieved growth inhibition by salt stress. Upshift of the external osmolality generated by either NaCl or sorbitol caused an increased spermidine transport with about 30- 40% increase at 10 mosmol/kg upshift.