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 and 6-mercaptopurine. 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 (By similarity). Up-regulated by adenine, via the adenine-dependent riboswitch. Belongs to the major facilitator superfamily. DHA1 family. PbuE (TC 2.A.1.2.25) subfamily.