Excretion of putrescine by the putrescine-ornithine antiporter encoded by the potE gene of Escherichia coli.

Excretion of putrescine from Escherichia coli was assessed by measuring its uptake into inside-out membrane vesicles. The vesicles were prepared from wild-type E. coli or E. coli transformed with plasmids containing one of the three polyamine transport systems. The results indicate that excretion of putrescine is catalyzed by the putrescine transport protein, encoded by the potE gene located at 16 min on the E. coli chromosome. Loading of ornithine (or lysine) inside the vesicles was essential for the uptake of putrescine, indicating that the protein exchanges putrescine and ornithine (or lysine) by an antiport mechanism. The Km and Vmax values for the putrescine uptake by inside-out membrane vesicles were 73 microM and 0.82 nmol/min per mg of protein, respectively. The antiport protein (potE protein) also catalyzed putrescine-putrescine and ornithine-ornithine exchange. The transport activity was not disturbed by inhibitors of energy production such as KCN and carbonyl cyanide m-chlorophenylhydrazone. When intact E. coli was used instead of the inside-out membrane vesicles, excretion of putrescine was also catalyzed by the antiport protein in the presence of ornithine in the medium.