Putrescine-binding protein probably required for putrescine uptake into cells. Binds putrescine with high affinity, spermidine with relatively low affinity. Does not bind cadaverine or spermine. Putrescine binding induces large inter-domain conformational changes. Could be present extracellularly due to cell outer membrane leakage. Belongs to the bacterial solute-binding protein PotD/PotF family.