Involved in the efflux of purine ribonucleosides, such as guanosine, adenosine and inosine, as well as purine bases guanine, adenine and hypoxanthine, and purine base analogs 2,6-diaminopurine, 6-mercaptopurine and 2-fluoroadenine. Therefore plays a role in maintaining the cellular purine base pools and protecting cells against toxic purine base analogs. Modulates expression of the purR and G-box regulons. Up-regulated by adenine, via the adenine-dependent riboswitch. Also up-regulated by hypoxanthine and guanine. Belongs to the major facilitator superfamily. DHA1 family. PbuE (TC 2.A.1.2.25) subfamily.