Putrescine-binding protein probably required for putrescine uptake into cells. Binds putrescine with high affinity (KD=3 nM), spermidine with relatively low affinity (KD=6.8 uM). Does not bind cadaverine or spermine. Putrescine binding induces large inter-domain conformational changes. No change in type 3 secretion system (T3SS) induction. Belongs to the bacterial solute-binding protein PotD/PotF family.