Catalyzes a salvage reaction resulting in the formation of AMP, that is energically less costly than de novo synthesis. May contribute to the recycling of adenine into adenylate nucleotides and the inactivation of cytokinins by phosphoribosylation. Possesses low activity toward adenine, but can efficiently convert cytokinins from free bases (active form) to the corresponding nucleotides (inactive form). AMP + diphosphate = 5-phospho-alpha-D-ribose 1-diphosphate + adenine Purine metabolism; AMP biosynthesis via salvage pathway; AMP from adenine: step 1/1. Homodimer. A number of isoforms are produced. According to EST sequences. No visible phenotype under normal growth conditions. Belongs to the purine/pyrimidine phosphoribosyltransferase family.