Uptake of allantoin into the cell (PubMed:11344136, PubMed:26967546). Allantoin uptake is not dependent on sodium, and PucI is likely to be a proton-coupled symporter (PubMed:26967546). Shows highest recognition for binding of allantoin, good recognition for binding of hydantoin, L-5-benzylhydantoin and 5-hydroxyhydantoin, and to a lesser extent for a range of nucleobases and nucleosides (PubMed:26967546). (S)-allantoin(in) + H(+)(in) = (S)-allantoin(out) + H(+)(out) Expression is very low in excess nitrogen (glutamate plus ammonia) and is induced during limiting-nitrogen conditions (glutamate). Expression is further induced when allantoin or allantoate are added during limiting-nitrogen conditions. Belongs to the purine-cytosine permease (2.A.39) family.