Heterocyclic nitrogen compounds may serve as nitrogen sources or nitrogen transport compounds in plants that are not able to fix nitrogen. This family represents ureide permease, a transporter of a wide spectrum of oxo derivatives of heterocyclic nitrogen compounds, including allantoin, uric acid and xanthine; it has 10 putative transmembrane domains with a large cytosolic central domain containing a 'Walker A' motif. Ureide permease is likely to transport other purine degradation products when nitrogen sources are low. Transport is dependent on glucose and a proton gradient. The family is found in bacteria, plants and yeast. These transporters are constituted of two sets of 5xTMs.